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
PR-URL: #64049
Refs: https://github.com/nodejs/reliability/issues?q=%22test-output-lcov-reporter%22
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

Author: trivikr

test/common/assertSnapshot.js

@@ -235,13 +235,30 @@ function replaceJunitDuration(str) {
 // This transform picks only the first line and then the lines from the test
 // file.
 functionpickTestFileFromLcov(str){
+constexpectedFile='output.js';
 constlines=str.split(/\n/);
 constfirstLineOfTestFile=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>'}`,
+);
+}
+
 constlastLineOfTestFile=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');
 constfixtures=require('../../../common/fixtures');
 constspawn=require('node:child_process').spawn;
 
-spawn(
+constchild=spawn(
 process.execPath,
 [
 '--no-warnings',
@@ -14,3 +14,16 @@ spawn(
 ],
 {stdio: 'inherit'},
 );
+
+child.on('error',(err)=>{
+throwerr;
+});
+
+child.on('exit',(code,signal)=>{
+if(signal){
+process.kill(process.pid,signal);
+return;
+}
+
+process.exitCode=code??1;
+});