RFR: TestDocComments fixes

[Maurizio Cimadamore]

This PR contains a port for:

https://github.com/openjdk/jextract/commit/0aaf75cdfe14df6f3a3f4e3d80f5384414a78ade

As well as a fix that is required by the changes in: https://git.openjdk.org/jextract/pull/137

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

Commit messages:
 - (jdk22 port) 7903435: TestDocComments fails on Windows
 - Fix snippets

Changes: https://git.openjdk.org/jextract/pull/140/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=140&range=00
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jextract/pull/140.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/140/head:pull/140

PR: https://git.openjdk.org/jextract/pull/140