test-fuse: improve/broaden applicability checks

test-fuse: improve/broaden applicability checks

The 'fuse' group doesn't exist on (most?) modern distros
anymore, so checking for it and membership results in not
being able to run the tests.

Additionally, on some systems (e.g. Fedora) the wrong fuse
module is installed (there are at least two competing ones
in python with the name 'fuse'), so run 'bup fuse -h' at
the beginning of the test just to do the import checks.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: rebase onto WVSKIP addition]
[rlb@defaultvalue.org: adjust commit summary]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>