Add test showing idempotency

test plan:
```
$ TEST_FILE=lua/tests/setup_spec.lua ./lua/tests/scripts/run_tests.sh
========================================
Testing: 	lua/tests/setup_spec.lua
Success	||	lean.setup Does not crash when loaded twice

Success: 	1
Failed : 	0
Errors : 	0
========================================
```

Author: 4e554c4c

lua/tests/setup_spec.lua

@@ -0,0 +1,10 @@
+local helpers = require('tests.helpers')
+local fixtures = require('tests.fixtures')
+
+describe('lean.setup', function()
+  it('Does not crash when loaded twice', function()
+    require('lean').setup {}
+    require('lean').setup {}
+  end)
+
+end)