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/+/2664246Reviewed-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>