Add support for running only affected tests during pull requests

[jverkoey]

We should easily be able to do a bazel query to identify which tests are affected during a pull request. This will allow us to only build/test targets that are affected by the PR.

For continuous integration we should still always run every test.

• Get the list of files that have changed as part of the PR.
• Run each file through a bazel query invocation.
• Aggregate the affected targets into a list.
• Only build/test the affected targets.

[jverkoey]

The following invocation will run tests for affected files:

    git diff --name-only "$TARGET_BRANCH...$KOKORO_GIT_COMMIT" | \
      while read line ; do bazel query "tests(rdeps(//components/..., $line))" 2>/dev/null ; done | \
      tee /dev/tty | \
      xargs bazel $ACTION --xcode_version $xcode_version $extra_args $verbosity_flags "${POSITIONAL[@]:2}"