tests: disable some tests for FreeBSD

tests: disable some tests for FreeBSD

Unfortunately, FreeBSD doesn't support sparse files in their tmpfs,
and they generlaly don't mount a tmpfs on /tmp anyway.  As a result
certain tests will either OOM kill a FreeBSD (if tmpfs is in use) or
will take forever (if it is not in use).

So let's turn off some tests for FreeBSD (m_hugefile is disabled on
MacOS already, for similar reasons).  We need to find a better
solution in the long term, but for now, these tests are guaranteed to
be a disaster on FreeBSD, so suppress them for now.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>