RFR: 7904141: JTReg should support running compact source files [v3]

[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:

  Add failing self-tests for `agent` VM mode

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/313/files
  - new: https://git.openjdk.org/jtreg/pull/313/files/d362d93c..e813217f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=02
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=313&range=01-02

  Stats: 17 lines in 1 file changed: 15 ins; 0 del; 2 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