test defs: get name of TAP-using tests correctly

test defs: get name of TAP-using tests correctly

This change fixes the names of the temporary directories used
by the TAP-based tests (e.g., test `foo.tap' now uses `foo.dir'
rather than `foo.tap.dir'); more importantly, this make the
variable `$destdir' defined in `tests/defs' usable also in the
TAP tests.

($me): Also strip the any `.tap' suffix from the name of the
test.