RFR: 8349369: test/docs/jdk/javadoc/doccheck/checks/jdkCheckLinks.java did not report on missing man page files

[Nizar Benalla]

Please review this small update to make the test more robust, the test did not always check links to other files if they did not contain a fragment or a `#`symbol.

TIA

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Commit messages:
 - update link checker to verify that files exist

Changes: https://git.openjdk.org/jdk/pull/23441/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23441&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349369
  Stats: 16 lines in 3 files changed: 7 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/23441.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23441/head:pull/23441

PR: https://git.openjdk.org/jdk/pull/23441