tests/main/xdg-open-compat: fix the test to do as advertised
The xdg-open-compat test has been broken since the core snap started shipping
xdg-open that actually talks to io.snaocraft.Launcher via `snapctl user-open`.
Because of this, the actual com.canonical.SafeLauncher interface was never
accessed, and the test ended up just redoing what xdg-open test does.
Attempt to fix the test by calling com.canonical.SafeLauncher diirectly via
dbus-send.
Also, snapd-xdg-open does not allow help: scheme, make sure that we account for
that in the test.
Signed-off-by: Maciej Borzecki <email address hidden>