Limit gmirror failpoint tests to the test worker

Limit gmirror failpoint tests to the test worker

This avoids injecting errors into the test system's mirrors.

gnop seems like a good solution here but it injects errors at the wrong
place vs where these tests expect and does not support a 'max global count'
like the failpoints do with 'n*' syntax.

Reviewed by:	cem, vangyzen
Sponsored by:	Dell EMC Isilon