Added more tests

tests/programs/sub.q

@@ -0,0 +1,9 @@
+main() {
+	assert 0 - 0 == 0
+	assert 1 - 1 == 0
+	assert 1 - 2 == -1
+	assert 2 - 1 == 1
+	assert 2 - 2 == 0
+	assert 4 - 2 == 2
+	assert 8 - 2 == 6
+}