Allow writing shell integration tests

[katre]

As part of bazelbuild/rules_android#76, I wanted to write shell integration tests (like android_sdk_integration_test.sh) for the new Starlark android_sdk_repository rule.

However, to do that with rules_bazel_integration_test I need two things:

1. A copy of bashunit, so script with (the trying to use the copy in Bazel is far too heavyweight), and
2. A way to directly depend on a bazel binary.

See https://github.com/katre/rules_android/commit/056dfc21eb075155c86f51734833b3e91554129d for a demo of using these in action. I am happy to update documentation in the repository as needed.

The license attribution is directly copied from the existing Bazel files. Please let me know if this is a problem.

[katre]

@illicitonion If you'd like to chat about this on slack or via email instead of github comments, let me know.

[cgrindel]

@katre Thanks for the PR. If I understand correctly, there are two parts to this PR:

1. Make bashunit available for Bazel tests.
2. Add an alias for the current Bazel version.

For the first part, perhaps we should consider creating a Bazel module repository for bashunit. Then, anybody can install it.

With regard to the alias, I took a look at how it was being used in rules_android. I think that we already support a pattern that may work. By any chance, did you come across the dynamic workspace test runner example? If you use the bazel_integeration_test macro and pass it a runner, the runner will be called with two environment variables set: BIT_BAZEL_BINARY and BIT_WORKSPACE_DIR. The BIT_BAZEL_BINARY is the path to the Bazel binary specified in the bazel_integration_test declaration.

I see in the Android SDK test that the test workspace is generated by the script. We support this pattern, as well, with one caveat. A test runner script can create all of the workspace files that it needs and then execute tests. The closest example is the use_create_scratch_dir_test.sh. (Sorry that is very cumbersome name. 😳) Just ignore the use of create_scratch_dir. The test is just creating a directory and filling it with Bazel workspace files. The caveat is that there is no good way to use bazel_integration_test without pointing it at an existing child workspace directory. However, I think that we can make that happen without too much effort.

Do you think using bazel_integration_test will work for your use case?