This reverts commit 98cc9e86.

Reason for revert: Breaks test isolation:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20builder/builds/29746

Original change's description:
> [test] Creating command before execution phase.
> 
> Immutable command class with shell, flags and
> environment.
> 
> Command creation moved from worker to the main
> process. Because of that there is no need to send
> test cases beyond process boundaries and load test
> suites in worker processes.
> 
> Bug: v8:6917
> Change-Id: Ib6a44278095b4f7141eb9b96802fe3e8117678a6
> Reviewed-on: https://chromium-review.googlesource.com/791710
> Commit-Queue: Michał Majewski <majeski@google.com>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49746}

TBR=machenbach@chromium.org,sergiyb@chromium.org,majeski@google.com

Change-Id: I44b99468d18fd093833f4185dad067a9eeaf2bc1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6917
Reviewed-on: https://chromium-review.googlesource.com/800292Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49747}