RFR: 7903981: Support easy way to use current class name for test actions

[Christian Stein]

Please review this change to allow an easier way to use current class name for test actions.

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Commit messages:
 - Compute main class name by parsing the Java source file
 - Fix simple class name computation
 - Omit package computation
 - Add tag specification entry
 - Add tag specification entry
 - 7906981: Support easy way to use current class name for test actions

Changes: https://git.openjdk.org/jtreg/pull/257/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=257&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903981
  Stats: 48 lines in 6 files changed: 29 ins; 2 del; 17 mod
  Patch: https://git.openjdk.org/jtreg/pull/257.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/257/head:pull/257

PR: https://git.openjdk.org/jtreg/pull/257