[jdk20] RFR: 8298785: gc/TestFullGCCount.java fails with "invalid boolean value: `null' for expression `vm.opt.ExplicitGCInvokesConcurrent'"

[Kim Barrett]

Please review this fix to the change to gc/TestFullGCCount.java made in
JDK-8298296.  I got the syntax for the new @requires condition wrong, and
while it worked fine for the cases I tested, there are other cases encountered
in our CI where it doesn't work.

`!vm.opt.ExplicitGCInvokesConcurrent` => `vm.opt.ExplicitGCInvokesConcurrent == true`
The problem is that if the option isn't provided at all then the vm.opt
variable is null rather than true or false.  I previously only tested with
explicit true or false for that option.  Oops.

`vm.gc == "G1"` => `vm.gc.G1`
The problem is that if the GC is defaulted (not explicitly "G1") and
-XX:+ExplicitGCInvokesConcurrent then the test will be run when it shouldn't
be, and will fail as per JDK-8298296.

Testing:
Hand checked the following configurations, with the expected results:
<no options> => passed
`-XX:+UseG1GC` => passed
`-XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent` => skipped
`-XX:+UseG1GC -XX:-ExplicitGCInvokesConcurrent` => passed
`-XX:+ExplicitGCInvokesConcurrent` => skipped
`-XX:-ExplicitGCInvokesConcurrent` => passed
`-XX:+UseParallelGC -XX:+ExplicitGCInvokesConcurrent` => passed

Currently waiting for mach5 testing.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk20/pull/34/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298785
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk20/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/34/head:pull/34

PR: https://git.openjdk.org/jdk20/pull/34