RFC: accept a bats_test rule

[BATS](https://bats-core.readthedocs.io/) is an integration test framework letting you write tests in Bash.
It's commonly used to test Bash scripts, though of course the language of the system-under-test can be anything.
Currently an implementation lives in bazel-lib: https://github.com/bazel-contrib/bazel-lib/blob/main/lib/private/bats.bzl

Perhaps it would be more discoverable in rules_shell?