Support coverage for `py_test`

[hwright]

bazel coverage should generate a coverage report for the python tests it runs. Ideally, it gathers the list of dependencies, and then computes coverage over that for a given test. When multiple tests are run in the same Bazel invocation, their reports should be merged into a single one.

I'm dropping this here mostly as a python-specific version of bazelbuild/bazel#1118. I know that the py_test rule is currently implemented natively in Bazel, and the implementation in rules_python just forwards there, but this is fundamentally a python issue, so this repo feels appropriate.

The general plan is to modify the python test runner within Bazel to use coverage.py to generate reports when running under coverage. I suspect it is a journey fraught with peril.

Alternatively, if it is possible to invoke coverage.py in ways other than modifying core Bazel, I'm open to suggestions.

[mattmoor]

@damienmg assigning to you to redirect appropriately.

[hwright]

fwiw, I've also been poking at this as documented here: https://groups.google.com/d/msg/bazel-discuss/LC97zBrMG84/cfVNdus-AwAJ

It's been slow going, as I'm pretty unfamiliar with Bazel internals, and haven't been able to get the ear of somebody who is.

[brandjon]

Setting P4 because this isn't a blocker for 1.0 support.

[JoshSullivan11]

Has this feature been implemented yet? If so, in what release version? It seems like coverage has been able to cover every other bit of our code repo except for all python files which is makes up most of our code base. Please let me know!