RFR: 7903683 Document how to run individual self-tests

[Ludvig Janiuk]

Add documentation on how to run individual self-tests with their ".ok" files to building.md.

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Commit messages:
 - document ok files

Changes: https://git.openjdk.org/jtreg/pull/184/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=184&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903683
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/184.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/184/head:pull/184

PR: https://git.openjdk.org/jtreg/pull/184