RFR: 8241596: ZGC: Shorten runtime of gc/z/TestUncommit.java

[Per Liden]

The test gc/z/TestUncommit.java is dangerously close to the 120 second 
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

Bug: https://bugs.openjdk.java.net/browse/JDK-8241596
Webrev: http://cr.openjdk.java.net/~pliden/8241596/webrev.0

/Per