Prior to this commit `make test` would run tests against all support
Debian distros.  As of this commit, `make test` runs tests against a
default Debian distro (currently buster, what deployment.eqiad.wmnet
is running), and `make test-all` will run tests against all supported
distros.  This makes `make test` more useful for development.

Updated README.rst accordingly.

Change-Id: I1cee774167cf6badc442cc85a0ab3185bbb10f58