RFR: 8315680: java/lang/ref/ReachabilityFenceTest.java should run with -Xbatch

[Gergö Barany]

This test requires certain methods to be compiled, but without `-Xbatch` the compiler races against the test code, which can lead to intermittent failures.

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Commit messages:
 - 8315680: java/lang/ref/ReachabilityFenceTest.java should run with -Xbatch

Changes: https://git.openjdk.org/jdk/pull/16023/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16023&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315680
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16023.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16023/head:pull/16023

PR: https://git.openjdk.org/jdk/pull/16023