Add trailing slash to test exemption dir paths (#4034)

In `GithubWebhookSubscription._isTestExempt` we exempt a collection of directories (in both the framework and the engine) from tests by checking if the filename starts with the directory path. This adds a trailing slash to any such checks.

Since `git` only stores files (not directories), all such paths will include a trailing slash. This avoids accidentally exempting directories such as `impeller/fixtures_that_actually_ship` when we only intended to exempt `impeller/fixtures`. Alright, bad example, but perhaps someday someone would have added something more ambiguous.

app_dart/lib/src/request_handlers/github/webhook_subscription.dart

@@ -454,19 +454,19 @@ class GithubWebhookSubscription extends SubscriptionHandler {
         filename.endsWith('.md') ||
         // Exempt paths.
         filename.startsWith('dev/devicelab/lib/versions/gallery.dart') ||
-        filename.startsWith('dev/integration_tests') ||
+        filename.startsWith('dev/integration_tests/') ||
         filename.startsWith('docs/') ||
         filename.startsWith('${engineBasePath}docs/') ||
         // ↓↓↓ Begin engine specific paths ↓↓↓
         filename == 'DEPS' || // note: in monorepo; DEPS is still at the root.
         isBuildPythonScript ||
-        filename.startsWith('${engineBasePath}impeller/fixtures') ||
-        filename.startsWith('${engineBasePath}impeller/golden_tests') ||
-        filename.startsWith('${engineBasePath}impeller/playground') ||
-        filename.startsWith('${engineBasePath}shell/platform/embedder/tests') ||
-        filename.startsWith('${engineBasePath}shell/platform/embedder/fixtures') ||
-        filename.startsWith('${engineBasePath}testing') ||
-        filename.startsWith('${engineBasePath}tools/clangd_check');
+        filename.startsWith('${engineBasePath}impeller/fixtures/') ||
+        filename.startsWith('${engineBasePath}impeller/golden_tests/') ||
+        filename.startsWith('${engineBasePath}impeller/playground/') ||
+        filename.startsWith('${engineBasePath}shell/platform/embedder/tests/') ||
+        filename.startsWith('${engineBasePath}shell/platform/embedder/fixtures/') ||
+        filename.startsWith('${engineBasePath}testing/') ||
+        filename.startsWith('${engineBasePath}tools/clangd_check/');
   }
 
   bool _isFusionEnginePath(String path) => (path.startsWith('engine/') || path == 'DEPS');

app_dart/test/request_handlers/github/webhook_subscription_test.dart

@@ -770,6 +770,36 @@ void main() {
       );
     });
 
+    test('Framework labels PRs, comment if no tests. Verify a trailing slash was added to the path prefix', () async {
+      const int issueNumber = 123;
+
+      tester.message = generateGithubWebhookMessage(
+        action: 'opened',
+        number: issueNumber,
+      );
+      when(pullRequestsService.listFiles(Config.flutterSlug, issueNumber)).thenAnswer(
+        (_) => Stream<PullRequestFile>.value(
+          PullRequestFile()..filename = 'testingénieux/davoir/trouvé/ce/truc',
+        ),
+      );
+
+      when(issuesService.listCommentsByIssue(Config.flutterSlug, issueNumber)).thenAnswer(
+        (_) => Stream<IssueComment>.value(
+          IssueComment()..body = 'some other comment',
+        ),
+      );
+
+      await tester.post(webhook);
+
+      verify(
+        issuesService.createComment(
+          Config.flutterSlug,
+          issueNumber,
+          argThat(contains(config.missingTestsPullRequestMessageValue)),
+        ),
+      ).called(1);
+    });
+
     test('Framework labels PRs, comment if no tests including hit_test.dart file', () async {
       const int issueNumber = 123;