Fix random unit-test failure for NVP advanced plugin

Fix random unit-test failure for NVP advanced plugin

Two problems were observed:
1. Tasks are not scheduled as desired in greenthread. The fix is to
   increase the max status probing time from 1 second to 10 seconds.
2. An error message is show in unit-test log output. Turns out in
   addition to a bug that may cause task manager stop prematurely,
   another unit-test case test_vcns_driver can cause same error
   message being displayed in the log.

Change-Id: I697dd8fc509308108ff1f40400f36ac6271bf4bb
Close-Bug: 1245698