test: the instdir-ltlib test is expected to fail.

test: the instdir-ltlib test is expected to fail.

Discussed in https://bugs.gnu.org/71596.

* t/list-of-tests.mk (handwritten_TESTS): move t/instdir-ltlib.sh
(XFAIL_TESTS): to here. It previously worked by accident
because lib_LTLIBRARIES was misspelled as lib_LTIBRARIES;
Paul Eggert fixed that on June 7,
commit 1d35638b23e95fe6f41c828a3442f6d7f242f4c4, and the
test started failing as a result.

* t/instdir-ltlib.sh: add some doc.
* t/all.sh: likewise. (Another xfail test, which is mysterious to me.)