swift234

[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.