coverage: Give each coverage test mode a separate output directory

When multiple test modes share the same test directory, this will allow them to
avoid interfering with each others' files and timestamps.

Author: Zalathar

src/tools/compiletest/src/common.rs

@@ -696,9 +696,16 @@ pub fn output_testname_unique(
     testpaths: &TestPaths,
     revision: Option<&str>,
 ) -> PathBuf {
+    // Coverage tests use the same test files for multiple test modes,
+    // so each mode should have a separate output directory.
+    let coverage_mode = match config.mode {
+        mode @ (CoverageMap | RunCoverage) => mode.to_str(),
+        _ => "",
+    };
     let mode = config.compare_mode.as_ref().map_or("", |m| m.to_str());
     let debugger = config.debugger.as_ref().map_or("", |m| m.to_str());
     PathBuf::from(&testpaths.file.file_stem().unwrap())
+        .with_extra_extension(coverage_mode)
         .with_extra_extension(revision.unwrap_or(""))
         .with_extra_extension(mode)
         .with_extra_extension(debugger)