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.)