Distribute test scripts even if we wouldn't run them

Distribute test scripts even if we wouldn't run them

This fixes a "make dist" tarball produced on a minimal system and run on a
non-minimal system. Automake knows that files that are only conditionally
included in dist_whatever_WHATEVER are to be distributed, but it does not
do the same for files that are only conditionally included in EXTRA_DIST,
which is how glib-tap.mk's various variables like dist_test_scripts work.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #557
Approved by: cgwalters