add discovery of tests in test_driver subdirectory

## Steps to Reproduce

Tests in test_driver subdirectory are not included under tests in Android Studio. According to [this flutter documentation](https://flutter.io/docs/testing#integration-testing) it seems the default location for integration tests.
Currently I need as workaround a directory symlink test -> test_driver to get them shown under tests in Android Studio.

## Version info

```
[✓] Flutter (Channel stable, v1.0.0, on Linux, locale en_US.UTF-8)
• Flutter version 1.0.0 at /mnt/ssd/flutter
• Framework revision 5391447fae (8 weeks ago), 2018-11-29 19:41:26 -0800
• Engine revision 7375a0f414
• Dart version 2.1.0 (build 2.1.0-dev.9.4 f9ebf21297)

[✓] Android toolchain - develop for Android devices (Android SDK 28.0.3)
• Android SDK at /mnt/ssd/android-sdk/
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• ANDROID_HOME = /mnt/ssd/android-sdk/
• Java binary at: /mnt/ssd/android-studio/jre/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)
• All Android licenses accepted.

[✓] Android Studio (version 3.3)
• Android Studio at /mnt/ssd/android-studio
• Flutter plugin version 32.0.1
• Dart plugin version 182.5124
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)

[✓] VS Code (version 1.30.2)
• VS Code at /usr/share/code
• Flutter extension version 2.21.1

[✓] Connected device (1 available)
• SM G920F • 02157df26097d81f • android-arm64 • Android 7.0 (API 24)

• No issues found!
```