This reverts commit 25457c60.

Reason for revert: https://crbug.com/v8/8731

Original change's description:
> [testrunner] load tests concurrently into test execution processor
> 
> 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}

TBR=machenbach@chromium.org,tmrts@chromium.org

Change-Id: I1e074a031dced367a32a93827b9e863b0331340f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8174
Reviewed-on: https://chromium-review.googlesource.com/c/1433792Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59065}