This CL migrates shared code from e2e, screenshots, and unit to a helper
file. It also introduces a unified runner for e2e and screenshots (and
perf once that's in place). Once this has landed I will remove
run_e2e.py and use run_test_suite.py instead.
Change-Id: I590e5eb6f3e8a4692767a2aa653b55685891614b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2054547
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Paul Lewis <aerotwist@chromium.org>