Assert that test names cannot contain dots

This is so that we can catch stray test output files, since test names
without dots can form predictable patterns we can match on.

src/tools/tidy/src/tests_revision_unpaired_stdout_stderr.rs

@@ -88,6 +88,12 @@ pub fn check(tests_path: impl AsRef<Path>, bad: &mut bool) {
                 continue;
             };
 
+            assert!(
+                !test_name.contains('.'),
+                "test name cannot contain dots '.': `{}`",
+                test.display()
+            );
+
             test_info.insert(test_name.to_string(), (test, expected_revisions));
         }