tests: skip unsupported tests on MacOS systems

tests: skip unsupported tests on MacOS systems

The "mkswap" program is not available on MacOS, so just use the
existing swap0.img.bz2 and swap1.img.bz2 files directly.

Because MacOS HFS+ doesn't support sparse files (welcome to the 80's)
the m_bigjournal test takes forever to zero out the whole 42GB test
filesystem.  Skip this test for Darwin kernels for now.

Unfortunately, neither "df -T" nor "stat -f -c %T" is available on
MacOS to directly determine the filesystem type, and I'm too lazy
to parse the output of "mount" and match it to the path of the test
directory in shell, so it just checks the kernel type and assumes
the filesystem type is HFS and skips the test.

Since this test runs on Linux the majority of the time, the loss of
test coverage is minimal.  If MacOS should ever get a real filesystem,
this can be revisited.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>