RFR: 7903683 Document how to run individual self-tests [v3]

[Ludvig Janiuk]

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

Ludvig Janiuk has updated the pull request incrementally with one additional commit since the last revision:

  accepting jaikiran's formulation

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/184/files
  - new: https://git.openjdk.org/jtreg/pull/184/files/8d10fcae..3674b067

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=184&range=02
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=184&range=01-02

  Stats: 9 lines in 1 file changed: 2 ins; 1 del; 6 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