the running test cases are not actively killed

Suppose we have only one test to run, which will stuck forever, for whatever reason.

Running the gtest-parallel by assigning `--timeout=x` will be also stuck forever, instead of finished in time and reporting that test to be timed out.

The problem occurs because we wait the process to be finished without a time out:

https://github.com/google/gtest-parallel/blob/f4d65b555894b301699c7c3c52906f72ea052e83/gtest_parallel.py#L92

A solution might be https://stackoverflow.com/a/10012262/4597218