RFR: 7904141: JTReg should support running compact source files [v6]

[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 one additional commit since the last revision:

  Environment variable already exported

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/313/files
  - new: https://git.openjdk.org/jtreg/pull/313/files/60d4fe45..0aeccd6d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=05
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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