Add not passing integration test (#154)

Add not passing integration test

Author: komaeda

tests/fixture/failure/testNotPassed.rs

@@ -0,0 +1,4 @@
+#[test]
+fn not_passing() {
+    assert!(false);
+}

tests/integration_tests.rs

@@ -76,6 +76,16 @@ fn run_single_test_failure() {
         .code(1);
 }
 
+#[test]
+fn run_single_test_not_passed() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["r", "testNotPassed.rs"])
+        .current_dir("tests/fixture/failure/")
+        .assert()
+        .code(1);
+}
+
 #[test]
 fn run_single_test_no_filename() {
     Command::cargo_bin("rustlings")