test: improve lcov reporter snapshot diagnostics

test/common/assertSnapshot.js

@@ -234,13 +234,30 @@ function replaceJunitDuration(str) {
 // This transform picks only the first line and then the lines from the test
 // file.
 function pickTestFileFromLcov(str) {
+  const expectedFile = 'output.js';
   const lines = str.split(/\n/);
   const firstLineOfTestFile = lines.findIndex(
-    (line) => line.startsWith('SF:') && line.trim().endsWith('output.js'),
+    (line) => line.startsWith('SF:') && line.trim().endsWith(expectedFile),
   );
+
+  if (firstLineOfTestFile === -1) {
+    assert.fail(
+      `Could not find LCOV source record ending with ${expectedFile} ` +
+      `in LCOV output:\n${str || '<empty>'}`,
+    );
+  }
+
   const lastLineOfTestFile = lines.findIndex(
     (line, index) => index > firstLineOfTestFile && line.trim() === 'end_of_record',
   );
+
+  if (lastLineOfTestFile === -1) {
+    assert.fail(
+      `Could not find end_of_record for LCOV source record ending with ${expectedFile} ` +
+      `in LCOV output:\n${str}`,
+    );
+  }
+
   return (
     lines[0] + '\n' + lines.slice(firstLineOfTestFile, lastLineOfTestFile + 1).join('\n') + '\n'
   );

test/fixtures/test-runner/output/lcov_reporter.js

@@ -3,7 +3,7 @@ require('../../../common');
 const fixtures = require('../../../common/fixtures');
 const spawn = require('node:child_process').spawn;
 
-spawn(
+const child = spawn(
   process.execPath,
   [
     '--no-warnings',
@@ -14,3 +14,16 @@ spawn(
   ],
   { stdio: 'inherit' },
 );
+
+child.on('error', (err) => {
+  throw err;
+});
+
+child.on('exit', (code, signal) => {
+  if (signal) {
+    process.kill(process.pid, signal);
+    return;
+  }
+
+  process.exitCode = code ?? 1;
+});