Skip dependent tests if we can't load loop or fuse

Skip dependent tests if we can't load loop or fuse

When modprobe is available (or when we know we're on Linux), try to load
the loop(back) and fuse modules before using them, and skip the relevant
tests if the load fails.  This allows the tests to proceed on systems
lacking those modules.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>