RFR: Fix javadoc generation and add check to testing suite

[James Yuzawa]

Motivation: There is bad javadoc being generated. This fixes that and also adds a check to the testing suite to make sure it does not happen again.

Notes: I branched off of jdk22 branch since I would like such fixes to make their way into the jdk22 branch. Please let me know if there is a different base or head I should use.

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

Commit messages:
 - Fix javadoc generation and add check to testing suite

Changes: https://git.openjdk.org/jextract/pull/227/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=227&range=00
  Stats: 30 lines in 9 files changed: 14 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jextract/pull/227.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/227/head:pull/227

PR: https://git.openjdk.org/jextract/pull/227