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.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
Merge branch 'travis-config' into travis/master-new-2