Allow optional context in on_exit callbacks.

[christhekeele]

Per #elixir-lang-core discussion, this PR allows ExUnit.Callbacks.on_exit/2 to be given a callback function that accepts an argument. This argument can be used to perform conditional cleanup based on test results.

In the discussion I proposed that this argument be an ExUnit.state(). In this implementation, however, I decided to allow it to be the full %ExUnit.Test{} | %ExUnit.TestModule{} struct, under the reasoning that:

• What the ExUnit.state() is for an %ExUnit.TestModule{} is somewhat unclear
  • I think it's always nil, and when passed along to a callback it loses context as to if a setup/1 test passed, or the callback was provided in setup_all/1
  • I considered instead passing either test.state or test_module.name along, and am still open to this approach
• This allows the callback author to decide how to handle the full context of setup/1 vs setup_all/1
  • Including writing a single re-usable function that handles both structs and can be used in both
• Should these structs expose more context beyond status in the future, the conditional cleanup we can do becomes more powerful
  • As a contrived example, a test could now use %ExUnit.Test{time: time_in_ms} to change how it does cleanup
  • It is worth discussing if this is a good thing or not

[sabiwara]

LGTM 💜