JvmMonitor: Handle more generation and collector scenarios. (#12469)

JvmMonitor: Handle more generation and collector scenarios. (#12469)

* JvmMonitor: Handle more generation and collector scenarios.

ZGC on Java 11 only has a generation 1 (there is no 0). This causes
a NullPointerException when trying to extract the spacesCount for
generation 0. In addition, ZGC on Java 15 has a collector number 2
but no spaces in generation 2, which breaks the assumption that
collectors always have same-numbered spaces.

This patch adjusts things to be more robust, enabling the JvmMonitor
to work properly for ZGC on both Java 11 and 15.

* Test adjustments.

* Improve surefire arglines.

* Need a placeholder