Add expected failures for ZFS

• :f_flag fails on ZFS because UF_IMMUTABLE isn't supported [1].
• :v_flag fails on ZFS because the mode for foo is [always] updated unnecessarily.

get_filesystem(..) (supporting function that was added to the test script)
is based on equivalent logic in usr.bin/extattr/tests/extattr_test.sh .

MFC after: 1 week
PR: 221189 [1], 221188 [2]