test: fix timing sensitivity in debugger test

test-debugger-util-regression.js was sensitive to timing, which seems
to have changed enough with V8 5.7 to cause this test to fail. Fix the
test to ensure we take debugger steps only at stable states instead of
erroneously taking a step on a partially complete buffer.

PR-URL: #11008
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-debugger-util-regression.js

@@ -4,6 +4,8 @@ const path = require('path');
 const spawn = require('child_process').spawn;
 const assert = require('assert');
 
+const DELAY = common.platformTimeout(200);
+
 const fixture = path.join(
   common.fixturesDir,
   'debugger-util-regression-fixture.js'
@@ -21,12 +23,16 @@ proc.stderr.setEncoding('utf8');
 
 let stdout = '';
 let stderr = '';
+proc.stdout.on('data', (data) => stdout += data);
+proc.stderr.on('data', (data) => stderr += data);
 
 let nextCount = 0;
 let exit = false;
 
-proc.stdout.on('data', (data) => {
-  stdout += data;
+// We look at output periodically. We don't do this in the on('data') as we
+// may end up processing partial output. Processing periodically ensures that
+// the debugger is in a stable state before we take the next step.
+const timer = setInterval(() => {
   if (stdout.includes('> 1') && nextCount < 1 ||
       stdout.includes('> 2') && nextCount < 2 ||
       stdout.includes('> 3') && nextCount < 3 ||
@@ -36,14 +42,14 @@ proc.stdout.on('data', (data) => {
   } else if (!exit && (stdout.includes('< { a: \'b\' }'))) {
     exit = true;
     proc.stdin.write('.exit\n');
+    // We can cancel the timer and terminate normally.
+    clearInterval(timer);
   } else if (stdout.includes('program terminated')) {
     // Catch edge case present in v4.x
     // process will terminate after call to util.inspect
     common.fail('the program should not terminate');
   }
-});
-
-proc.stderr.on('data', (data) => stderr += data);
+}, DELAY);
 
 process.on('exit', (code) => {
   assert.strictEqual(code, 0, 'the program should exit cleanly');