Jtreg doesn't properly handle putting the same test in multiple problem lists

If you have two problem lists in use and they both include an entry for the same test, only the entry from the first problem list mentioned is honored. For example, if you have a test in ProblemList.txt that is excluded only on macosx-x64 but is excluded on all platforms in ProblemList-zgc.txt, and you use JTREG_EXTRA_PROBLEM_LISTS=ProblemList-zgc.txt, the ProblemList-zgc.txt entry is ignored and the test will run on all platforms except macosx-x64.

See the following CR for an example where this is actually impacting us:

https://bugs.openjdk.org/browse/JDK-8270202?focusedId=14618207&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14618207

The test is excluded on all platforms in ProblemList-zgc.txt, but is only excluded for macox-x64 in ProblemList.txt, and since this is the first problem list mentioned, it is the entry that gets honored. Thus we end up running the test with ZGC even though we shouldn't be.