add --exit flag to dev/devicelab/bin/test_runner.dart (#134165)

Resolves #134070 

Adds a flag to the `test_runner.dart test` script that will cause the test runner to exit upon first failure (or, said another way, exit without retrying).

This is in parity with the `--exit` flag of `dev/devicelab/bin/run.dart`.

dev/devicelab/lib/command/test.dart

@@ -24,6 +24,11 @@ class TestCommand extends Command<void> {
           'settings in the test case, and will results in error if no device\n'
           'with given ID/ID prefix is found.',
     );
+    argParser.addFlag(
+      'exit',
+      help: 'Exit on the first test failure. Currently flakes are intentionally (though '
+            'incorrectly) not considered to be failures.',
+    );
     argParser.addOption(
       'git-branch',
       help: '[Flutter infrastructure] Git branch of the current commit. LUCI\n'
@@ -90,6 +95,7 @@ class TestCommand extends Command<void> {
       silent: (argResults!['silent'] as bool?) ?? false,
       useEmulator: (argResults!['use-emulator'] as bool?) ?? false,
       taskArgs: taskArgs,
+      exitOnFirstTestFailure: argResults!['exit'] as bool,
     );
   }
 }