py_test rule enable to use a test written in Python. A py_test is
basically a py_binary that returns a non null on failure. Extraneous
support is need to have nice output (see //src/test/shell/unittest.bash
for the kind of support neeeded).

Actually the py_test code was already there but it was just missing the
necessary glue code. Also added an integration test for py_* rules in
Bazel.

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