Only report failed test cases in scripted test

This change makes the scripted test runner report only the failed test
cases in the log file. This makes it easier to see what went wrong in
the test run.

yash-cli/tests/scripted_test.rs

@@ -62,7 +62,8 @@ where
     // is a failed test case. Check the log file to see if there is one.
 
     let log = std::fs::read_to_string(&log_file).unwrap();
-    assert!(!log.contains("FAILED"), "{}", log);
+    let failures = failures(&log);
+    assert!(failures.is_empty(), "{failures}");
 }
 
 /// Runs a test subject.
@@ -73,6 +74,42 @@ fn run(name: &str) {
     unsafe { run_with_preexec(name, || Ok(())) }
 }
 
+/// Extracts the failed test cases from the log file.
+fn failures(log: &str) -> String {
+    let mut lines = log.lines();
+    let mut test_case = Vec::new();
+    let mut result = String::new();
+
+    // Each test case in the log file is enclosed by the "%%% START: " and
+    // "%%% PASSED: " or "%%% FAILED: " lines. We extract lines between these
+    // markers and append them to the result string.
+    while let Some(start) = lines.find(|line| line.starts_with("%%% START: ")) {
+        test_case.clear();
+        test_case.push(start);
+        for line in lines.by_ref() {
+            if line.starts_with("%%% PASSED: ") {
+                // Discard this test case
+                break;
+            } else if line.starts_with("%%% FAILED: ") {
+                test_case.push(line);
+
+                // Add this test case to the result
+                for line in test_case.drain(..) {
+                    result.push_str(line);
+                    result.push('\n');
+                }
+                result.push('\n');
+
+                break;
+            } else {
+                test_case.push(line);
+            }
+        }
+    }
+
+    result
+}
+
 #[test]
 fn alias() {
     run("alias-p.sh")