RFR: 7903741: some test properties are not documented

[Jonathan Gibbons]

Please review a simple doc and test update for tag-spec.html, after it was recently noted that `test.module.path` was available but not documented.

The new test verifies 1:1 correspondence between names that are available, as env vars or system properties, and those that are documented.

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Commit messages:
 - CODETOOLS-7903741: some test properties are not documented

Changes: https://git.openjdk.org/jtreg/pull/203/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=203&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903741
  Stats: 81 lines in 3 files changed: 79 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/203.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/203/head:pull/203

PR: https://git.openjdk.org/jtreg/pull/203