RFR: 7904141: JTReg should support running compact source files [v4]

[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 incrementally with three additional commits since the last revision:

 - Clean up
 - Extract and reuse main method helper code
 - Add driver-based runs

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/313/files
  - new: https://git.openjdk.org/jtreg/pull/313/files/e813217f..c16f25a5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=03
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=02-03

  Stats: 226 lines in 7 files changed: 134 ins; 85 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