This permits using `unittest.suite` with test rules that have nondefault
attributes, while retaining compatibility with current usage.

For instance, this permits setting a `timeout` on each test in a
`unittest.suite`.  Previously, all tests in a `unittest.suite` would
have the default timeout, with no good way to alter this.  This
made it hard to eliminate all the warnings produced from using the
`--test_verbose_timeout_warnings` bazel option.

While timeouts were the motivation, the solution here is not specific
to timeouts. It will permit arbitrary additional arguments to the test
rules in a `unittest.suite`.

Fixes bazelbuild#98