RFR: 8263725: JFR oldobject tests are not run when GCs are specified explicitly

[Stefan Karlsson]

The tests are tagged with @requires == "null", which has the effect that if you run with GC explicitly set, like:
make -C ../build/fastdebug test TEST=test/jdk/jdk/jfr/event/oldobject/TestZ.java JTREG="JAVA_OPTIONS=-XX:+UseG1GC"

then the tests won't run. The intention has probably been to only run the test with the default GC. I propose that we remove this and run the tests with other GCs as well. 

I've tested this locally with ZGC, G1, Parallel and Serial.

The following test doesn't work with ZGC and has been disabled for that GC:
test/jdk/jdk/jfr/event/oldobject/TestObjectDescription.java

The following test doesn't work with Serial and has been disabled for that GC
test/jdk/jdk/jfr/event/oldobject/TestClassLoaderLeak.java

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Commit messages:
 - 8263725: JFR oldobject tests are not run when GCs are specified explicitly

Changes: https://git.openjdk.java.net/jdk/pull/3046/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3046&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263725
  Stats: 19 lines in 19 files changed: 0 ins; 14 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3046.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3046/head:pull/3046

PR: https://git.openjdk.java.net/jdk/pull/3046