ZGC: Shorten runtime of gc/z/TestUncommit.java

The test gc/z/TestUncommit.java is dangerously close to the 120 second default JTReg timeout, and spurious timeouts have been reported [1]. We should shorten the runtime of this test to avoid that.

[1] http://mail.openjdk.java.net/pipermail/zgc-dev/2020-March/000892.html