Merge branch 'testsuite-work' into master

At last!

Keeping the `master' and `testsuite-work' separated was becoming
quite artificial, since there is no way we are going to release
a 1.12 version of Automake that doesn't include the support for
the TAP test protocol and for user-defined test drivers -- which
are all implemented into the testsuite-work branch.

Moreover, keeping those two branches separated was causing more
maintenance and double-checking work than were warranted, was
making the automake git repository even more contorted, and, on
the top of all that, was actually *reducing coverage* by making
all the important changes in testsuite-work less visible (less
on-field testing by developers that routinely use bleeding edge
versions, less automated testing by services like Hydra/NixOS,
et cetera).