fix(stdio): drain stdio before process.exit (#244)

`process.exit()` may not flush stdio, which in turn can lead to the loss of output.
See nodejs/node#6456.

This issue manifests with `max-failures should work` test being flaky,
where we examine large output and it is sometimes truncated.

src/cli.ts

@@ -133,9 +133,20 @@ async function runTests(command: any) {
   const runner = new Runner(loader);
   const tagFilter = command.tag && command.tag.length ? command.tag : undefined;
   const result = await runner.run(!!command.list, testFiles, tagFilter);
+
+  // Calling process.exit() might truncate large stdout/stderr output.
+  // See /~https://github.com/nodejs/node/issues/6456.
+  //
+  // We can use writableNeedDrain to workaround this, but it is only available
+  // since node v15.2.0.
+  // See https://nodejs.org/api/stream.html#stream_writable_writableneeddrain.
+  if ((process.stdout as any).writableNeedDrain)
+    await new Promise(f => process.stdout.on('drain', f));
+  if ((process.stderr as any).writableNeedDrain)
+    await new Promise(f => process.stderr.on('drain', f));
+
   if (result === 'sigint')
     process.exit(130);
-
   if (result === 'forbid-only') {
     console.error('=====================================');
     console.error(' --forbid-only found a focused test.');