tests: avoid checking command result in cluster

tests: avoid checking command result in cluster

When running cluster test with clvmd, the actual 'monitoring'
happens in cluster - so the 'already monitored' message
is also logged within clvmd code and the command cannot
see such effect.

clvmd was incapable to report this information back to command
so it cannot be displayed this way.

Add 'lvs -o+seg_monitor' validation which also works in clustered mode.