Create t/force-delete from t/lib.sh force-delete and use it everywhere.

Move the t/lib.sh force-delete code to t/force-delete as a standalone
command, and use it everywhere -- particularly to delete t/tmp during
"make clean", given the recent move to run all tests inside t/ via
TMPDIR (18f838c9a66d521f9fa042eead3c5771ca99b03a).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Remove t/ from the test-meta.sh test tree now that it contains t/tmp/.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>