This CL adds a command line option to angle_end2end_tests that will
iterate over all test configs. For each config it'll fork a new child
process that will run only a single config. This will allow us to
isolate each config to a specific child process. Hopefully this will
reduce test flakiness due to driver issues with multiple configs.

The command line option is "--separate-process-per-config".

Note that there are about 25 configs right now.

Bug: angleproject:3393
Change-Id: Ia117b371bbe159c1b0d28d82befffeb0f40467a9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1591428
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>