Add helper modules for bats

According to these instructions:
https://bats-core.readthedocs.io/en/stable/tutorial.html#quick-installation

default.nix

@@ -20,8 +20,8 @@ resholve.mkDerivation {
   src = fs.toSource {
     root = ./.;
     fileset = fs.unions [
-      ./bin/git-check-assertions
-      ./test/git-check-assertions.bats
+      ./bin
+      ./test
     ];
   };