Add a leg_only test suite, and make sure to run it in our presubmit script.

frog/tests/leg_only/src/StatementsTest.dart

Index: frog/tests/leg_only/src/StatementsTest.dart
===================================================================
--- frog/tests/leg_only/src/StatementsTest.dart	(revision 0)
+++ frog/tests/leg_only/src/StatementsTest.dart	(working copy)
@@ -5,7 +5,6 @@
 void main() {
   print(3);
   print(4);
-  /*
   if (true) {
     print('hest');
   }
@@ -16,5 +15,4 @@
   }
   int foo() {}
   int i = 0;
-  */
 }