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.
Previously the mirage library/package wouldn't be built/installed
correctly due to missing deps which I now added as specified in the opam
file. While I was at it, I also enabled tests.