Functionally the same, but makes cherry picking simpler.

(Note from @antirez: modified to apply to 2.8)

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.

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

Pause the test with running instances available for state inspection on
error.

It is now possible to kill and restart sentinel or redis instances for
more real-world testing.

The 01 unit tests the capability of Sentinel to update the configuration
of Sentinels rejoining the cluster, however the test is pretty trivial
and more tests should be added.

Most units will start with these two basic tests to create an
environment where the real tests are ran.

Nothing tested at all so far... Just the infrastructure spawning N
Sentinels and N Redis instances that the test will use again and again.