This adds a reduced version of bot_default with only d8-based test suites.
Those can be used for gc stress testing, fuzzing and predictable mode.

This also adds a missing explicit build rule for the debugger test suite.
It's working as it's part of the isolate file, but incremental
re-isolation might break on code changes.

Running the tests is still commented out to allow the infra-side to
land. Afterwards, we'll activate the extra testing as a V8 CL
commenting the lines back in.

Bug: v8:7285
Change-Id: I7b758c212f0c4ae3d2f79beea5ec597cee479565
Reviewed-on: https://chromium-review.googlesource.com/864045Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50545}