Add native_image_test rule

[sgammon]

We should add a rule which matches functionality in the Gradle and Maven plugins for building test binaries with native-image. The behavior is not that different from the standard rule:

• We'll need to have reflection agent support
• We'll need to pass flags if we want test discovery to work (i.e. JUnit), and we do
• Coverage tooling should be instrumented as directed by Bazel, ideally with unified reporting
• Maybe this factors into plans for PGO

[sgammon]

relates to #220