Stringdedup tests are failing if the ergonomically select GC does not support it

If the ergonomically selected GC doesn't support string deduplication, all tests in gc/stringdedup are failing. This is the case for client VMs.

I think the problem is that the requires "vm.gc == "null"" basically means, that the test case should be executed if a GC has been selected ergonomically, but "gc.gc == "G1"" only means that, the G1 gc is present and can be selected. So the test case is executed (and fails) if, for instance, the ergo GC is Serial, and G1 is compiled into the JVM.

https://git.openjdk.java.net/jdk/blob/master/test/hotspot/jtreg/gc/stringdedup/TestStringDeduplicationAgeThreshold.java

Since certain GCs are required. I think the test case should be executed in othervm mode with the specific GC enabled, if the GC is available.