RFR: 8347531: The signal tests are failing after JDK-8345782 due to an unrelated warning

[Joakim Nordström]

Could I get a review of this fix for the SigTestDriver test?

This fix adds more context from the libjsig deprecation warning to only catch the correct scenario in the test.

Previously the test only checked for the words `VM warning` in the output to determine the deprecation warning was present. However, another VM Warning printed when running a flag on an unsupported platform made the SigTestDriver fail incorrectly (`VM warning: -XX:+UseLargePages not supported in this VM`).

## Testing
* [x] Verified test still succeeds in "normal" way
* [x] Verified test also succeeds despite having the other warning.
* [ ] Tested tier1-tier3

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Commit messages:
 - 8347531: The signal tests are failing after JDK-8345782 due to an unrelated warning

Changes: https://git.openjdk.org/jdk/pull/23103/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23103&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347531
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23103.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23103/head:pull/23103

PR: https://git.openjdk.org/jdk/pull/23103