loading every test up-front into the processing queue costs about 224MB for a
x64 testsuite run.

This CL eliminates that overhead by utilizing generators and threading.

LoadingProc now loads test after receiving the results of the loaded tests.

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

Bug: v8:8174
Change-Id: I8f4e6de38430c54fe126e4504b52851866769efb
Reviewed-on: https://chromium-review.googlesource.com/c/1420678
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59056}