RFR: CODETOOLS-7903101: provide better documentation for jtreg test infrastructure

[Jonathan Gibbons]

Please review some introductory documentation for the jtreg test infrastructure ... that is, the mechanisms used to test the jtreg tool itself, and its component parts.

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Commit messages:
 - CODETOOLS-7903101: provide better documentation for jtreg test infrastructure

Changes: https://git.openjdk.java.net/jtreg/pull/57/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=57&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903101
  Stats: 119 lines in 2 files changed: 119 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/57.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/57/head:pull/57

PR: https://git.openjdk.java.net/jtreg/pull/57