RFR: 7904141: JTReg should support running compact source files

[Christian Stein]

Please review this change that adds support for running compact source files as jtreg `main` tests.

This pull request also adds JDK 25 to the GHA-based CI workflow and updates to use newer GitHub actions: `actions/checkout at v6` and `actions/setup-java at v5`

-------------

Commit messages:
 - Cleanup
 - Fix syntax
 - Install JDK 25 and 21
 - Configure correct makefile
 - Fix path to pre-installed JDK 25
 - Introduce `JDK25HOME` variable
 - 7904141: JTReg should support running compact source files

Changes: https://git.openjdk.org/jtreg/pull/313/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904141
  Stats: 260 lines in 10 files changed: 249 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jtreg/pull/313.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/313/head:pull/313

PR: https://git.openjdk.org/jtreg/pull/313