This will let both the ANGLETest and dEQP test runners handle child
processes correctly. The existing ANGLETest runner will reuse displays,
and the dEQP test runner will write to a batch-unique filename.

This solves the problem of the multi-process --bot-mode writing to the
same file from multiple children simultaneously.

Long-term it will be good to include the dEQP QPA files into the test
artifacts.

Also disables flushing after every log write when running as a child
process. This hugely improves performance on machines with no SSD.

Test: https://chromium-swarm.appspot.com/task?id=4f2b87c4f8234910
Bug: angleproject:5157
Change-Id: I226d24adf55e0f8b98e5a8e7947862e6906b4c40
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2464424Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>