Avoid make distclean failing if t/tmp* matches a directory.

Avoid make distclean failing if t/tmp* matches a directory.

Some of the tests in t/ make directories with names generated by the test.pl
tempfile(), and if they exit abnormally without deleting them, this would cause
distclean to fail. So change `rm -f t/tmp*` to `rm -rf t/tmp*`

Remove other cruft from the same line in the Makefile.
t/misctmp* hasn't been generated since 7a7e49369367a634.
t/forktmp* hasn't been generated since 1c25d394345c1b97.
t/io/fs.t has deleted t/c itself since 44a8e56aa037ed0f.
(Whilst it might abort and fail to delete it, there's no special treatment
in the Makefile for the other file that it creates, "t/TEST$$")