The first validation command was actually failing but the test was not,
because only the last return value is taken into account, obviously. Add
a '&&' between the commands.
Also test against the built files, not the templates (in particular
because `appstream-util` doesn't like .in.in templates, and anyway it's
always better to test against the finale file).
Finally move to "validate-relax" test for the time being.
"validate-strict" actually makes a bunch of errors, but I can't make the
time to look at these now. Let's just settle with basic validation at
least.