RFR: CODETOOLS-7903261: Add FAQ entry about the time taken to run tests

[Jonathan Gibbons]

Please review an update to the FAQ, to add an entry about the information recorded in `.jtr` files about test execution times.

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Commit messages:
 - ODETOOLS-7903261: Add FAQ entry about the time taken to run tests

Changes: https://git.openjdk.org/jtreg/pull/106/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=106&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903261
  Stats: 56 lines in 1 file changed: 53 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/106.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/106/head:pull/106

PR: https://git.openjdk.org/jtreg/pull/106