V8 test suites return tests by loading them into memory up-front. Up-front
loading has memory and loading-time overhead for the testrunner.

This CL converts the test-suites to load tests during the test run in parallel.

After CL, most test suites start running immediately and all test suites start
running after 3 seconds on a modern SSD.

This CL eliminates the 200 MiB memory spikes that causes memory problems in mac
minis.

Overhead of running test262 before CL:
Line #    Mem usage     Increment   Line Contents
=================================================
   264  225.760 MiB 195.8.000 MiB         tests = self._load_testsuite_generators(args, options)
-------------------------------------------------
Overhead of running test262 after CL:
Line #    Mem usage    Increment   Line Contents
================================================
   264   28.840 MiB    0.000 MiB         tests = self._load_testsuite_generators(args, options)
-------------------------------------------------

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

Bug: v8:8174, v8:8552, v8:8728
Change-Id: Iab540b9410239b05dc80b4a5228db25d8b8fb3b9
Reviewed-on: https://chromium-review.googlesource.com/c/1454478
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@{#59408}