Add tests for evaluator

The evaluator is currently accompanied by test cases.

Let's add a set of tests that can be run on Source Academy
playground.

To facilitate testing, let's use a global variable to
track the most recent result from `parse_and_eval`.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "source-test"]
+	path = source-test
+	url = https://github.com/arsalanc-v2/source-test.git

README.md

@@ -44,3 +44,11 @@ const f = amb(1, 2, 3); const g = amb(5, 6, 7); g;
 // Result: 5, 6, 7, 5, 6, 7, 5, 6, 7
 ```
 
+### Running Tests
+
+1. Copy the code from the following files into the playground:
+    * `evaluator.js`
+    * `test.js`
+    * `source-test/main.js`
+
+2. Remove one of the two instances of the repeated function `variable_declaration_name` (from `evaluator.js` and `source-test/main.js`)