RFR: 7904141: JTReg should support running compact source files [v2]

[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`

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Restrict support of JEP 512 features to JDK 25 and higher
  
  https://openjdk.org/jeps/512

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/313/files
  - new: https://git.openjdk.org/jtreg/pull/313/files/f4f5e00d..d362d93c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=00-01

  Stats: 179 lines in 4 files changed: 102 ins; 70 del; 7 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