TestSuite has a static method LoadTestSuite that should properly configure the
TestSuite instance (i.e. loaded status files and tests), however the method
leaves some configuration logic to the caller.

The leaky abstraction causes the caller to do a bunch of loading operations (see
the removed methods in base_runner.py).

This CL isolates the TestSuite loading logic to the static method only.

This is a refactoring only change without any intended logical changes.

R=machenbach@chromium.org
CC=​​​yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8174
Change-Id: I105059c9c9e050f03bb584174e2bd7ceeae2b228
Reviewed-on: https://chromium-review.googlesource.com/c/1396417
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58744}