Makefile: test and test-all targets

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.

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Related MRs:

• !301 (merged)
• !314 (merged) (This MR)
• !315 (merged)
• master

Depends-On: !315 (merged)