Add the following flag to the test runner to aid debugging:

--delay-test-start=<number of seconds>
  This delays the start of each test by <number of seconds>. This is
useful when using debuggers, like RenderDoc, that need some extra time
to attach to the process before capturing data.

Bug: angleproject:5312
Change-Id: Ie1aab155e780dfae7311e046af0a7702904cd9eb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2522504Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>