Running "angle_unittests" with --bot-mode would produce noisy
test groupings for each permutation of parameterized tests. Fix
this by only splitting configs that begin with "ES". e.g. ES2_Vulkan,
ES3_OpenGL, etc.

Bug: angleproject:3162
Change-Id: I0b0ebe1c9cef9cbbce0489a2bc3f62761d43a1c7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419046
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>