Add an explicit ignore message for "up-to-date" tests

When running tests without the `--force-rerun` flag, compiletest will
automatically skip any tests that (in its judgement) don't need to be run again
since the last time they were run.

This patch adds an explicit reason to those skipped tests, which is visible
when running with `rust.verbose-tests = true` in `config.toml`.

Author: Zalathar

src/tools/compiletest/src/lib.rs

@@ -808,8 +808,11 @@ fn make_test(
                 &config, cache, test_name, &test_path, src_file, revision, poisoned,
             );
             // Ignore tests that already run and are up to date with respect to inputs.
-            if !config.force_rerun {
-                desc.ignore |= is_up_to_date(&config, testpaths, &early_props, revision, inputs);
+            if !config.force_rerun
+                && is_up_to_date(&config, testpaths, &early_props, revision, inputs)
+            {
+                desc.ignore = true;
+                desc.ignore_message = Some("up-to-date");
             }
             test::TestDescAndFn {
                 desc,