test: move timer-dependent test to sequential

`test-regress-GH-897` is dependent on a timer firing within a period of
time. Especially on some of the FreeBSD hosts on CI, we have seen tests
like that fail when run in parallel. (This may have nothing to do with
FreeBSD and may just mean that the hosts are resource-constrained.) Move
this test to sequential as we have done with several other
timer-dependent tests recently.

The test has also been refactored and documented via comments.

PR-URL: https://github.com/nodejs/node/pull/9487


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>