Add support for splitting large jvm based test suites

Created by: kageiit

This adds support for splitting up jvm test suites with a large number of tests.

This can be used effectively on CI/machines with a lot of cores to run an existing large test suite in parallel by bucketing the test classes evenly. We have seen our test suite speedup from 1 hour down to 20 min by using this change.

This change usually accompanies a definition like

test_bucket_size = 200


original_java_test = java_test
def java_test(
    name,
    srcs=[],
    **kwargs
    ):
  splits = (len(srcs) + test_bucket_size - 1) / test_bucket_size
  if splits > 1:
    for part in xrange(splits):
      n_name = name if part == 0 else '{}_{}'.format(name, part + 1)
      original_java_test(
        name=n_name,
        srcs=srcs,
        splits=splits,
        part=part+1,
        **kwargs
      )

When running on CI