clippy: fix test filtering when TESTNAME is empty

Author: RalfJung

tests/compile-test.rs

@@ -144,8 +144,17 @@ impl TestContext {
         let target_dir = PathBuf::from(var_os("CARGO_TARGET_DIR").unwrap_or_else(|| "target".into()));
         let mut config = Config {
             output_conflict_handling: error_on_output_conflict,
+            // Pre-fill filters with TESTNAME; will be later extended with `self.args`.
             filter_files: env::var("TESTNAME")
-                .map(|filters| filters.split(',').map(str::to_string).collect())
+                .map(|filters| {
+                    filters
+                        .split(',')
+                        // Make sure that if TESTNAME is empty we produce the empty list here,
+                        // not a list containing an empty string.
+                        .filter(|s| !s.is_empty())
+                        .map(str::to_string)
+                        .collect()
+                })
                 .unwrap_or_default(),
             target: None,
             bless_command: Some(if IS_RUSTC_TEST_SUITE {