test: Have test_fs work with non-functional guestmount tools

Since 2011 Ubuntu has intentionally broken support for guestmount[1] by
default and requires sysadmin intervention to re-enable support.  This
in turn exposed that in our tests if guestmount is available but fails
we do not fall back to trying to use sudo.  Restructure our code to try
sudo if guestmount fails rather than only when it is not in our path.
Further, only note that we are using fuse on success of the call.

[1]: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725



Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>