tests: Check error indicator before checking boolean result
If a GLib function that raises an error fails unexpectedly during testing, it's a lot more informative to see an assertion failure that includes details of the error, rather than just the boolean result.
For a conventional GLib function we should not look at the "out" parameters until we have checked that the function was successful.
I noticed this mistake while adding more tests, for !93 (merged).
/cc @jpwhiting @denittis