RFR: 8266056: runtime/stringtable/StringTableCleaningTest.java failed with "RuntimeException: Missing Callback in [10, 11]"

[Per Liden]

This patch addresses two issues with the StringTableCleaningTest test.

1) The test doesn't fully take into account that a concurrent GC might not complete (it can abort if the VM wants to terminate). findCallback() should return -1 if no callbacks are found, instead of failing the test.

2) The regexp patterns used for ZGC needs to be more exact. The end pattern now also matches aborted GCs, which it shouldn't. This leads to "Garbage Collection (*) Aborted" being incorrectly matched as a GC end.

This patch also makes sure the output from the test VM ends up in the test log, otherwise it's impossible to tell what went wrong if this test fails in the future.

This patch also removes the test from the ProblemList.

Testing:
* Passed Tier1-3.
* Manual testing of StringTableCleaningTest with various GCs.

-------------

Commit messages:
 - 8266056: runtime/stringtable/StringTableCleaningTest.java failed with "RuntimeException: Missing Callback in [10, 11]"

Changes: https://git.openjdk.java.net/jdk/pull/3815/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3815&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266056
  Stats: 8 lines in 2 files changed: 3 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3815.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3815/head:pull/3815

PR: https://git.openjdk.java.net/jdk/pull/3815