Add helper modules for bats

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

.gitmodules

@@ -0,0 +1,9 @@
+[submodule "test/test_helper/bats-support"]
+	path = test/test_helper/bats-support
+	url = https://github.com/bats-core/bats-support.git
+[submodule "test/test_helper/bats-assert"]
+	path = test/test_helper/bats-assert
+	url = https://github.com/bats-core/bats-assert.git
+[submodule "test/test_helper/bats-file"]
+	path = test/test_helper/bats-file
+	url = https://github.com/bats-core/bats-file.git

README.md

@@ -13,6 +13,12 @@ You include a small bash script inside your commit messages, and `git-check-asse
 
 ⚠️ Only run this on repositories and branches that you trust, since the `bash` scripts in the commit messages can do whatever they want.
 
+## Building
+
+```bash
+nix build .?submodules=1
+```
+
 ## Examples of commit messages
 
 Assert that a commit builds:

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
     ];
   };
 

test/git-check-assertions.bats

@@ -1,10 +1,14 @@
 #!/usr/bin/env bats
 
 setup() {
+  load 'test_helper/bats-support/load'
+  load 'test_helper/bats-assert/load'
+
   DIR="$( cd "$( dirname "$BATS_TEST_FILENAME" )" >/dev/null 2>&1 && pwd )"
   PATH="$DIR/../bin:$PATH"
 }
 
 @test "can run" {
-  git-check-assertions
+  run git-check-assertions
+  assert_success
 }

test/test_helper/bats-assert

@@ -0,0 +1 @@
+Subproject commit 697471b7a89d3ab38571f38c6c7c4b460d1f5e35

test/test_helper/bats-file

@@ -0,0 +1 @@
+Subproject commit 6bee58bec7c2f4aed1a7425ccd4bdc42b4a84599

test/test_helper/bats-support

@@ -0,0 +1 @@
+Subproject commit 0954abb9925cad550424cebca2b99255d4eabe96