gplazma: Fix JVM implementation dependency in unit test

The ListCommand outputs plugins in hash table order - this ordering is
implementation dependent, which in turn causes the
ListCommandTests.testDetailedList unit test to fail with some JVMs (eg Oracle
JDK 8).

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7196/
(cherry picked from commit c0a68c5a29d48d06dcb10ead9261aadeac074c6d)