Test that auxiliaries are built against their own directives

There are probably plenty of tests that will check this incidentally, but it's
convenient to have a dedicated self-test.

Author: Zalathar

tests/ui/compiletest-self-test/aux-has-props.rs

@@ -0,0 +1,14 @@
+//@ edition: 2024
+//@ aux-build: aux_with_props.rs
+//@ compile-flags: --check-cfg=cfg(this_is_aux)
+//@ run-pass
+
+// Test that auxiliaries are built using the directives in the auxiliary file,
+// and don't just accidentally use the directives of the main test file.
+
+extern crate aux_with_props;
+
+fn main() {
+    assert!(!cfg!(this_is_aux));
+    assert!(aux_with_props::aux_directives_are_respected());
+}

tests/ui/compiletest-self-test/auxiliary/aux_with_props.rs

@@ -0,0 +1,6 @@
+//@ edition: 2024
+//@ compile-flags: --cfg=this_is_aux
+
+pub fn aux_directives_are_respected() -> bool {
+    cfg!(this_is_aux)
+}