test: add common.mustCall() to stream test

Refs: https://github.com/nodejs/node/pull/30561/files#r348667256

PR-URL: #30561
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-stream-writable-clear-buffer.js

@@ -3,7 +3,7 @@
 // This test ensures that the _writeableState.bufferedRequestCount and
 // the actual buffered request count are the same.
 
-require('../common');
+const common = require('../common');
 const Stream = require('stream');
 const assert = require('assert');
 
@@ -24,12 +24,12 @@ const testStream = new StreamWritable();
 testStream.cork();
 
 for (let i = 1; i <= 5; i++) {
-  testStream.write(i, () => {
+  testStream.write(i, common.mustCall(() => {
     assert.strictEqual(
       testStream._writableState.bufferedRequestCount,
       testStream._writableState.getBuffer().length
     );
-  });
+  }));
 }
 
 testStream.end();