RFR: 8270021: Incorrect log decorators in gc/g1/plab/TestPLABEvacuationFailure.java

[Albert Mingkun Yang]

Fixed the log decorator, and added checks for non-empty statistics.

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Commit messages:
 - plab

Changes: https://git.openjdk.java.net/jdk/pull/4707/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4707&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270021
  Stats: 12 lines in 2 files changed: 4 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4707.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4707/head:pull/4707

PR: https://git.openjdk.java.net/jdk/pull/4707