RFR: 8293828: JFR: jfr/event/oldobject/TestClassLoaderLeak.java still fails when GC cycles are not happening

[Paul Hohensee]

Trivial change to get a test to run reliably on Shenandoah and ZGC.

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

Commit messages:
 - 8293828: JFR: jfr/event/oldobject/TestClassLoaderLeak.java still fails when GC cycles are not happening

Changes: https://git.openjdk.org/jdk/pull/10319/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10319&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293828
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10319.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10319/head:pull/10319

PR: https://git.openjdk.org/jdk/pull/10319