[Android] Add testing support

This adds support for running tests for the stdlib built
for Android, both on the host machine and on an Android device.

The Android variant of Swift may be built and tested using
the following `build-script` invocation:

```
$ utils/build-script \
  -R \                                           # Build in ReleaseAssert mode.
  -T \                                           # Run all tests.
  --android \                                    # Build for Android.
  --android-deploy-device-path /data/local/tmp \ # Temporary directory on the device where Android tests are run.
  --android-ndk ~/android-ndk-r10e \             # Path to an Android NDK.
  --android-ndk-version 21 \
  --android-icu-uc ~/libicu-android/armeabi-v7a/libicuuc.so \
  --android-icu-uc-include ~/libicu-android/armeabi-v7a/icu/source/common \
  --android-icu-i18n ~/libicu-android/armeabi-v7a/libicui18n.so \
  --android-icu-i18n-include ~/libicu-android/armeabi-v7a/icu/source/i18n/
```

See docs/Testing.rst for more details.

utils/android/adb_test_runner.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python
+# adb_test_runner.py - Calls adb_test_runner.main -*- python -*-
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+import sys
+
+from adb_test_runner.main import main
+
+
+if __name__ == '__main__':
+    sys.exit(main())