Fix failures when l3_agent_scheduler ext is not available

Some plugins do not support the L3 agent scheduler extension.
Therefore, make the tests that rely on it dependent on whether
this is actually available in the plugin under test.

Change-Id: I3154b646677379a34465c6826b71d4ad72c471a2
Closes-bug: #1231152
Merge "Fix failures when l3_agent_scheduler ext is not available"