Prior to the application of this patch, `testharness.js` would correctly
report tests that used the `force_timeout` method as "timed out,"
(ignoring subsequent erros), but it would only do so after the
harness-defined "timeout" period had elapsed. This behavior was in
direct conflict with the documentation, which described its effect as
"immediate" [1].

Update the implementation to expose the documented behavior.

[1]

> Occasionally tests may have a race between the harness timing out and
> a particular test failing; typically when the test waits for some
> event that never occurs. In this case it is possible to use
> `test.force_timeout()` in place of `assert_unreached()`, to
> immediately fail the test but with a status of `TIMEOUT`. This should
> only be used as a last resort when it is not possible to make the test
> reliable in some other way.