fusefs: annotate deliberate file descriptor leaks in the tests

closing a file descriptor causes FUSE activity that is superfluous to the
purpose of most tests, but would nonetheless require matching expectations.
Rather than do that, most tests deliberately leak file descriptors instead.
This commit moves the leakage from each test into two trivial functions:
leak and leakdir. Hopefully Coverity will only complain about those
functions and not all of their callers.

Sponsored by: The FreeBSD Foundation