test(date): validate locale formats via runtime expansion

Replace literal format code checks with validation of expanded output.
Fixes test failures on macOS 15.7.2 where nl_langinfo returns
composite format codes instead of explicit ones.

Fixes #9654

Author: naoNao89

src/uu/date/src/locale.rs

@@ -112,6 +112,28 @@ mod tests {
     cfg_langinfo! {
         use super::*;
 
+        /// Helper function to expand a format string with a known test date
+        ///
+        /// Uses a fixed test date: Monday, January 15, 2024, 14:30:45 UTC
+        /// This allows us to validate format strings by checking their expanded output
+        /// rather than looking for literal format codes.
+        fn expand_format_with_test_date(format: &str) -> String {
+            use jiff::civil::date;
+            use jiff::fmt::strtime;
+
+            // Create test timestamp: Monday, January 15, 2024, 14:30:45 UTC
+            let test_date = match date(2024, 1, 15).at(14, 30, 45, 0).in_tz("UTC") {
+                Ok(zoned) => zoned,
+                Err(_) => return String::new(),
+            };
+
+            // Expand the format string with the test date
+            match strtime::format(format, &test_date) {
+                Ok(expanded) => expanded,
+                Err(_) => String::new(),
+            }
+        }
+
         #[test]
         fn test_locale_detection() {
             // Just verify the function doesn't panic
@@ -121,10 +143,30 @@ mod tests {
         #[test]
         fn test_default_format_contains_valid_codes() {
             let format = get_locale_default_format();
-            assert!(format.contains("%a")); // abbreviated weekday
-            assert!(format.contains("%b")); // abbreviated month
-            assert!(format.contains("%Y") || format.contains("%y")); // year (4-digit or 2-digit)
-            assert!(format.contains("%Z")); // timezone
+            let expanded = expand_format_with_test_date(&format);
+
+            // Verify expanded output contains expected components
+            // Test date: Monday, January 15, 2024, 14:30:45
+            assert!(
+                expanded.contains("Mon") || expanded.contains("Monday"),
+                "Expanded format should contain weekday name, got: {}", expanded
+            );
+
+            assert!(
+                expanded.contains("Jan") || expanded.contains("January"),
+                "Expanded format should contain month name, got: {}", expanded
+            );
+
+            assert!(
+                expanded.contains("2024") || expanded.contains("24"),
+                "Expanded format should contain year, got: {}", expanded
+            );
+
+            // Keep literal %Z check - this is enforced by ensure_timezone_in_format()
+            assert!(
+                format.contains("%Z"),
+                "Format string must contain %Z timezone (enforced by ensure_timezone_in_format)"
+            );
         }
 
         #[test]
@@ -135,25 +177,34 @@ mod tests {
             // The format should not be empty
             assert!(!format.is_empty(), "Locale format should not be empty");
 
-            // Should contain date/time components
-            let has_date_component = format.contains("%a")
-                || format.contains("%A")
-                || format.contains("%b")
-                || format.contains("%B")
-                || format.contains("%d")
-                || format.contains("%e");
-            assert!(has_date_component, "Format should contain date components");
-
-            // Should contain time component (hour)
-            let has_time_component = format.contains("%H")
-                || format.contains("%I")
-                || format.contains("%k")
-                || format.contains("%l")
-                || format.contains("%r")
-                || format.contains("%R")
-                || format.contains("%T")
-                || format.contains("%X");
-            assert!(has_time_component, "Format should contain time components");
+            let expanded = expand_format_with_test_date(&format);
+
+            // Verify expanded output contains date components
+            // Test date: Monday, January 15, 2024
+            let has_date_component = expanded.contains("15")     // day
+                || expanded.contains("Jan")                      // month name
+                || expanded.contains("January")                  // full month
+                || expanded.contains("Mon")                      // weekday
+                || expanded.contains("Monday");                  // full weekday
+
+            assert!(
+                has_date_component,
+                "Expanded format should contain date components, got: {}", expanded
+            );
+
+            // Verify expanded output contains time components
+            // Test time: 14:30:45
+            let has_time_component = expanded.contains("14")     // 24-hour
+                || expanded.contains("02")                       // 12-hour
+                || expanded.contains("30")                       // minutes
+                || expanded.contains(":")                        // time separator
+                || expanded.contains("PM")                       // AM/PM indicator
+                || expanded.contains("pm");
+
+            assert!(
+                has_time_component,
+                "Expanded format should contain time components, got: {}", expanded
+            );
         }
 
         #[test]