Add tests for install-plugins.sh

Use bats-assert and bats-support for testing
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..6f8a2f8
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,6 @@
+[submodule "tests/test_helper/bats-support"]
+	path = tests/test_helper/bats-support
+	url = https://github.com/ztombol/bats-support
+[submodule "tests/test_helper/bats-assert"]
+	path = tests/test_helper/bats-assert
+	url = https://github.com/ztombol/bats-assert