RFR: 7904088: Include the timestamp to the log message when a timeout is fired

[Jaikiran Pai]

Can I please get a review of this trivial change which includes the timestamp for a log message when a test action times out?

Before this change, jtreg would log a message like:

> Timeout signalled after 120 seconds

Now it will also include a timestamp for that log message. This should help during investigations of timing out tests:

> [08:10:07.344] Timeout signalled after 120 seconds

The change is a one liner in `src/share/classes/com/sun/javatest/regtest/agent/Alarm.java`. Rest of the changes are mere refactoring to allow for using the same `DateTimeFormat` consistently in a couple of other places.

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Commit messages:
 - add timestamp to log

Changes: https://git.openjdk.org/jtreg/pull/293/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=293&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904088
  Stats: 67 lines in 5 files changed: 48 ins; 15 del; 4 mod
  Patch: https://git.openjdk.org/jtreg/pull/293.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/293/head:pull/293

PR: https://git.openjdk.org/jtreg/pull/293