Add test case for missing return statement

semantic/missing_return_statement.invalid.mj

+class MissingReturnStatement {
+    public static void main(String[] args) {
+        MissingReturnStatement obj = new MissingReturnStatement();
+        boolean x = obj.test(5, 0);
+    }
+
+    public boolean test(int a, int b) {
+        if (a < b) {
+            return false;
+        }
+    }
+}