Test Runner: Fix default config grouping. 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>