test for preserve exit status code

the use of the status code was included in a45d2c8, but the test was missed out so rebasing

tested with: cargo test

tests/integration_buck2.rs

@@ -25,3 +25,13 @@ fn test_buck2_specific_version() {
     assert!(stdout.starts_with("buck2 "), "found {}", stdout);
     assert.success();
 }
+
+#[test]
+fn test_buck2_fail() {
+    let mut cmd = Command::cargo_bin("buckle").unwrap();
+    cmd.arg("--totally-unknown-argument");
+    let assert = cmd.assert();
+    let stderr = String::from_utf8(assert.get_output().stderr.to_vec()).unwrap();
+    assert!(stderr.contains("error: Found argument"), "found {}", stderr);
+    assert.failure();
+}