This loads each test's testcfg.py as a unique module rather than flattening all into testcfg. Other than accessing LoadTestSuite there should be no references into testcfg files.

Bug: v8:6375
Change-Id: If863c1b35096b2589111e8091bb7d68f135da674
Reviewed-on: https://chromium-review.googlesource.com/498807Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45178}