test: improve lcov reporter snapshot diagnostics

Propagate failures from the nested LCOV reporter fixture process and
make missing LCOV records fail with a targeted assertion instead of
comparing against blank transformed output.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr

test/common/assertSnapshot.js

@@ -238,9 +238,25 @@ function pickTestFileFromLcov(str) {
   const firstLineOfTestFile = lines.findIndex(
     (line) => line.startsWith('SF:') && line.trim().endsWith('output.js'),
   );
+
+  if (firstLineOfTestFile === -1) {
+    assert.fail(
+      'Could not find coverage for test/fixtures/test-runner/output/output.js ' +
+      `in LCOV output:\n${str || '<empty>'}`,
+    );
+  }
+
   const lastLineOfTestFile = lines.findIndex(
     (line, index) => index > firstLineOfTestFile && line.trim() === 'end_of_record',
   );
+
+  if (lastLineOfTestFile === -1) {
+    assert.fail(
+      'Could not find end_of_record for test/fixtures/test-runner/output/output.js ' +
+      `in LCOV output:\n${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;
+});