RFR: 8315696: SignedLoggerFinderTest.java test failed

[Sean Coffey]

Update the recently added LoggerFinder tests to cater for a possible condition where the test finishes before the boot logger executor service has flushed its pending data.

By simulating a slow thread in the ExecutorService used in BootstrapLogger, I was able to reproduce the issue described. Adding the `BootstrapLoggerUtils.awaitPending();` call allows the tests to pass.

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Commit messages:
 - refactory testlibrary name
 - 8315696

Changes: https://git.openjdk.org/jdk/pull/15571/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15571&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315696
  Stats: 147 lines in 8 files changed: 74 ins; 64 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/15571.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15571/head:pull/15571

PR: https://git.openjdk.org/jdk/pull/15571