We don't have any tests which run multiple isolates concurrently and
starts a profiler in each of them. This test is a basic starting point
so that we can check for flakiness caused by races or interrupts.

The profiling mechanisms should be totally separate for two isolates,
so this should (theoretically) not cause any problems.

A use case for multiple isolates is for workers or in Node via cloud
functions, so we should get some more coverage here.

Change-Id: I0ca6d1296bc7bae7238c51b4487259d09e38d690
Reviewed-on: https://chromium-review.googlesource.com/c/1309823
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57207}