RFR: 8318455: Make the compiler/ ... /SharedTrampolineTest.java require C2 explicitly

[Evgeny Nikitin]

The test seem to only work with C2. A previous change to the test, the JDK-8314838, allowed for passing JVM arguments to the child process. However, if those parameters change the compiler used, the test fails.

This PR makes this requirement (to have C2 as compiler) explicit.

Testing: runs with '-XX:TieredStopAtLevel=3' with old version (failed), fixed version (skipped) and with new version but no additional arguments (passed).

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Commit messages:
 - 8318455: Make the compiler/sharedstubs/SharedTrampolineTest.java require C2 explicitly

Changes: https://git.openjdk.org/jdk/pull/16258/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16258&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318455
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16258.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16258/head:pull/16258

PR: https://git.openjdk.org/jdk/pull/16258