RFR: 8359293: Make TestNoNULL extensible

[Doug Simon]

On Thu, 12 Jun 2025 09:49:14 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

> 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`)

Thanks for the review.

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PR Comment: https://git.openjdk.org/jdk/pull/25777#issuecomment-2969454924