Joining the thread from the watchdog is problematic, since e.g.
{pthread_join} (the implementation of {Thread::Join} on POSIX systems)
has undefined behaviour if multiple threads try to join at the same
time. In practice, this leads to deadlocks.

Thus implement termination by just calling {TaskRunner::Terminate}, but
not {TaskRunner::Join}. This fixes the deadlocks in the inspector
fuzzer.
The inspector test binary is fixed simarly, even though there it seems
to not cause problems so far.

In both files, the {Terminate} function is inlined into callers because
it's only a single line now, with one to two users.

Also, replace the single fuzzer test (which is invalid javascript) by
two tests: One called "invalid" explicitly, still with invalid
javascript, and one empty file, which is valid input. That one
reproduced the deadlock.

R=szuend@chromium.org

Bug: chromium:1142437
Change-Id: I8fb98b0cdbf3ceff6af6849397e5da5a4e9acd3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2526384Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71054}