Terminate flutter test when no longer needed in integration test. (flutter#159117)

Towards flutter#51421.

```sh
flutter_tools % dart test test/integration.shard/break_on_framework_exceptions_test.dart

02:38 +29: All tests passed!

54692 ttys003    0:00.02 /opt/homebrew/bin/zsh -il
```

Requires flutter#159115 for the process
cleanup to work properly, but this is safe to land as-is, otherwise we
still accumulate `flutter` processes over and over as each test case
runs which is not WAI.

Author: matanlurey

packages/flutter_tools/test/integration.shard/break_on_framework_exceptions_test.dart

@@ -33,23 +33,29 @@ void main() {
 
     final FlutterTestTestDriver flutter = FlutterTestTestDriver(tempDir);
 
-    await _timeoutAfter(
-      message: 'Timed out launching `flutter test`',
-      work: () => flutter.test(withDebugger: true, pauseOnExceptions: true),
-    );
+    try {
+      await _timeoutAfter(
+        message: 'Timed out launching `flutter test`',
+        work: () => flutter.test(withDebugger: true, pauseOnExceptions: true),
+      );
 
-    await _timeoutAfter(
-      message: 'Timed out waiting for VM service pause debug event',
-      work: flutter.waitForPause,
-    );
+      await _timeoutAfter(
+        message: 'Timed out waiting for VM service pause debug event',
+        work: flutter.waitForPause,
+      );
 
-    int? breakLine;
-    await _timeoutAfter(
-      message: 'Timed out getting source location of top stack frame',
-      work: () async => breakLine = (await flutter.getSourceLocation())?.line,
-    );
+      int? breakLine;
+      await _timeoutAfter(
+        message: 'Timed out getting source location of top stack frame',
+        work: () async => breakLine = (await flutter.getSourceLocation())?.line,
+      );
 
-    expect(breakLine, project.lineContaining(project.test, exceptionMessage));
+      expect(breakLine, project.lineContaining(project.test, exceptionMessage));
+    } finally {
+      // Some of the tests will quit naturally, and others won't.
+      // By this point we don't need the tool anymore, so just force quit.
+      await flutter.quit();
+    }
   }
 
   testWithoutContext('breaks when AnimationController listener throws', () async {