RFR: 8359293: Make TestNoNULL extensible

[Doug Simon]

There are some closed markdown files that contain "NULL". These should be excluded from checking by TestNoNULL. This PR makes TestNoNull extensible in terms of exclusions with the following system properties:

* `excludedTestExtensions` - command separated list of file extensions (e.g. `-DexcludedTestExtensions=.md,.txt`)
* `sourceExclusions` - command separated list of file paths relative to repo root (e.g. `-DsourceExclusions=src/hotspot/share/prims/jvmti.xml`)
* `testExclusions` - command separated list of file paths relative to repo root (e.g. `-DtestExclusions=test/hotspot/jtreg/vmTestbase/nsk/share/jvmti/README`)

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Commit messages:
 - made TestNoNULL extensible in terms of exclusions

Changes: https://git.openjdk.org/jdk/pull/25777/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25777&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359293
  Stats: 19 lines in 1 file changed: 14 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25777.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25777/head:pull/25777

PR: https://git.openjdk.org/jdk/pull/25777