RFR: 7904141: JTReg should support running compact source files [v5]

[Christian Stein]

> Please review this change that adds support for running [JEP 512: Compact Source Files and Instance Main Methods](https://openjdk.org/jeps/512) 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 with a new target base due to a merge or a rebase. The pull request now contains 13 commits:

 - Merge branch 'master' into CODETOOLS-7904141-compact-source-files
 - Clean up
 - Extract and reuse main method helper code
 - Add driver-based runs
 - Add failing self-tests for `agent` VM mode
 - Restrict support of JEP 512 features to JDK 25 and higher
   
   https://openjdk.org/jeps/512
 - Cleanup
 - Fix syntax
 - Install JDK 25 and 21
 - Configure correct makefile
 - ... and 3 more: https://git.openjdk.org/jtreg/compare/da8892fb...60d4fe45

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Changes: https://git.openjdk.org/jtreg/pull/313/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=04
  Stats: 345 lines in 12 files changed: 337 ins; 0 del; 8 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