Test Runner: Pick single/multi-process mode automatically. This enables multi-process when running with more than one test config. This saves developers from remembering to add the arg. It can also simplify our test configurations by allowing us to omit the default arguments. On Android by default we stay with single process. It also changes the --bot-mode flag into --single-process and --multi-process which override the default behaviour. Bug: angleproject:5585 Change-Id: I3dd885a8a8e6604c224693c546ff5d15224e016e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2664246 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>