checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
The upstream repository location changed, the old one redirects to the
new one.
It enables tests, which were disabled previously due to them requiring a
submodule, because they are useful to check if all dependencies are
working correctly.
