Mitigate a racy test

A worker cannot observe a peer-failed message sent before it started listening.

test-tap/fixture/fail-fast/multiple-files/fails.cjs

@@ -1,13 +1,20 @@
 const test = require('../../../../entrypoints/main.cjs');
 
+// Allow some time for all workers to launch.
+const grace = new Promise(resolve => {
+	setTimeout(resolve, 500);
+});
+
 test('first pass', t => {
 	t.pass();
 });
 
-test('second fail', t => {
+test('second fail', async t => {
+	await grace;
 	t.fail();
 });
 
-test('third pass', t => {
+test('third pass', async t => {
+	await grace;
 	t.pass();
 });