test: Add test case for MODULE_NOT_FOUND.

test/simple/test-require-exceptions.js

@@ -31,3 +31,12 @@ assert.throws(function() {
 assert.throws(function() {
   require(common.fixturesDir + '/throws_error');
 });
+
+// Requiring a module that does not exist should throw an
+// error with its `code` set to MODULE_NOT_FOUND
+assert.throws(function () {
+  require(common.fixturesDir + '/DOES_NOT_EXIST');
+}, function (e) {
+  assert.equal('MODULE_NOT_FOUND', e.code);
+  return true;
+});