Add helper modules for bats

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

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
 }