When an overall timeout is reached, swarming sends a SIGTERM to
terminate the test runner. The test runner has a signal handler on the
main process to terminate all workers gracefully.

Additionally, every worker process installs a signal handler for
terminating ongoing tests wrapped by command.Command.

Also, command.Command is used on the main process to list tests for
cctest and gtest executables, which led to overriding the test runner's
main signal handler.

This CL disables using signal handlers in commands by default and only
explicitly enables it in safe source locations.

Bug: v8:8292
Change-Id: Ifceadaff75bdd2b77e761498bccbe00b6a3e265c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2002528Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65815}