Improve distribution of Autotest testsuite dependencies.

A fresh distribution tarball includes a generated testsuite, but
does not have the benefit of rules from automatic dependency tracking.

This means if a file included by the testsuite is modified in the
tarball, the build will not detect that testsuite is out of date
and will not try to rebuild it.

A similar scenario occurs if 'make distclean' is run after building
the test suite.

Since modern Automake uses make rules to generate the dependency stubs,
we can use the distributed testsuite dependencies record to produce
an accurate initial list of prerequisites.  And by ensuring that this
file is updated whenever the testsuite is built, we can handle the
'make distclean' case too.