bug#9299: testsuite maintainer check rules growing stale

[Stefano Lattarini]

Many maintainer checks in the top-level Makefile.am assume that all
the test scripts match the wildcard `$(srcdir)/*.test'.  With the
introduction of TAP-based tests (which have a `.tap' extension) and
with the increasing use of helper shell scripts used/sourced by the
tests (this is done avoid code duplication), that assumption is
starting to hold less an less.  So we should devise a better way to
match all (and preferibly only) the files to which the tests-specific
maintainer checks are to be applied.

Regards,
  Stefano