RFR: 8373654: Test sources/TestNoNull should only run once

[David Holmes]

This test is checking for backsliding on the use of NULL in the hotspot source files. The test only needs to be run once per change but it is defined as a regular tier1 test which then gets run for each os-arch combination, plus product and debug, and then again with a bunch of flags in tier3. This is just a waste of resources. At a minimum we should make the test flagless so that it doesn't run in tier3, but we can also restrict it to a single platform.

Before the change the test ran 27 times in tier1-3 in our CI. After it runs once. 

Thanks

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Commit messages:
 - 8373654: Test sources/TestNoNull should only run once

Changes: https://git.openjdk.org/jdk/pull/28818/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373654
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28818.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28818/head:pull/28818

PR: https://git.openjdk.org/jdk/pull/28818