RFR: 8285965: TestScenarios.java does not check for "<!-- safepoint while printing -->" correctly

[Christian Hagedorn]

This is another rare occurrence of `` that is not handled correctly by `TestScenarios.java`. 

We wrongly search this safepoint message in the test VM output with `getTestVMOutput()`:

https://github.com/openjdk/jdk/blob/9c2548414c71b4caaad6ad9e1b122f474e705300/test/hotspot/jtreg/testlibrary_tests/ir_framework/tests/Utils.java#L44-L53

But this does not help since the IR matcher is parsing the `hotspot_pid` file for IR matching and not the test VM output. We could therefore find this safepoint message in the `hotspod_pid` file and bail out of IR matching while the test VM output does not contain it. This lets `TestScenarios.java` fail. 

The fix we did for other IR framework tests is to redirect the output of the JTreg test VM itself to a stream in order to search it for ``. We are dumping this message as part of a warning when the IR matcher bails out:

https://github.com/openjdk/jdk/blob/9c2548414c71b4caaad6ad9e1b122f474e705300/test/hotspot/jtreg/compiler/lib/ir_framework/driver/irmatching/IRMatcher.java#L86-L96

Output for the reported failure:

Scenario #3 - [-XX:TLABRefillWasteFraction=53]:
[...]
Found , bail out of IR matching


I suggest to use the same fix for `TestScenarios`.

Thanks,
Christian

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Commit messages:
 - 8285965: TestScenarios.java does not check for "<!-- safepoint while printing -->" correctly

Changes: https://git.openjdk.java.net/jdk/pull/8647/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8647&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285965
  Stats: 38 lines in 2 files changed: 11 ins; 16 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8647.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8647/head:pull/8647

PR: https://git.openjdk.java.net/jdk/pull/8647