RFR: 7904067: Include the milli second part in the start and end times reported in each action's section

[Jaikiran Pai]

Can I please get a review of this trivial change which updates the reporting of start and end dates of each action to also include the milli second part? 

As noted in https://bugs.openjdk.org/browse/CODETOOLS-7904067, this detail can sometimes help during debugging when comparing other log files which typically have the milli second part.

Existing self tests continue to pass with this change.

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Commit messages:
 - 7904067: Include the milli second part in the start and end times reported in each action's section

Changes: https://git.openjdk.org/jtreg/pull/275/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=275&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904067
  Stats: 12 lines in 1 file changed: 5 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/275.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/275/head:pull/275

PR: https://git.openjdk.org/jtreg/pull/275