Add additional sections to the testing instructions; these are based
on the same sections in the Zuul repo. This brings the nodepool repo
to parity.
Change-Id: I7a1b2c62963a815a1ab864e3685be1653e5734fc
It looks like the nodepool README has lagged behind on updates
since the release of Zuul v3, so we update it with similar
content and also add a TESTING file with verified content.
Change-Id: I6923a532e3a702def0595998ea2fab5c067da2fe