tests: fix typo in tac-continue

tests: fix typo in tac-continue

* tests/misc/tac-continue: Fix typo in usually-skipped test:
s/mkfifo_or_skip/mkfifo_or_skip_/ (i.e., append "_").
This test is usually skipped, because I'm probably the only
one to set the FULL_PARTITION_TMPDIR envvar, and recently the
one I'd been using ceased to exist, so this test was skipped
even for me.  Good argument for making this a root-only test
and creating a full partition just for this test case.