Bug 1906191 - Change how ProcessHandler handles output_timeout....

Bug 1906191 - Change how ProcessHandler handles output_timeout. r=releng-reviewers,gbrown a=test-only

First and foremost, this undoes the patch for bug 1845125, which causes
other problems (e.g. output being dropped past a certain point).

The rest of the patch addresses bug 1845125 at the root: Ultimately, this
is a similar problem to bug 1863675.

wait is blocked on joining the stream reader thread, but the stream
reader thread is blocked on readline because the process is finished but
hasn't been waited on yet.

So joining the stream reader thread after the process is known to have
been finished is expected to work more reliably... except when a child
has been spun up that keeps it alive. In this case, we just keep the
reader thread working in the background, which is a similar strategy to
what was done in bug 1863675.

Differential Revision: https://phabricator.services.mozilla.com/D218389