Sentinel test: restart_instance should refresh pid attrib.

Also kill_instance was modified to warn when a test will try to kill the
same instance multiple times for error.
Sentinel test: test majority crashing Sentinels.

The test was previously performed by removing the master from the
Sentinel monitored masters. The test with the Sentinels crashed is
more similar to real-world partitions / failures.
Sentinel test: jump to next unit on test failure.

Sentinel tests are designed to be dependent on the previous tests in the
same unit, so usually we can't continue with the next test in the same
unit if a previous test failed.
Sentinel test: restart instances left killed by previous unit.

An unit can abort in the middle for an error. The next unit should not
assume that the instances are in a clean state, and must restart what
was left killed.