test: use regexp to confir error message

In test/parallel/test-stream-writable-null.js, use a regular expression
to validate error message in assert.throws() call.

PR-URL: #14268
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: bangwu

test/parallel/test-stream-writable-null.js

@@ -32,7 +32,7 @@ assert.doesNotThrow(() => {
 assert.throws(() => {
   const m = new MyWritable();
   m.write(false, (err) => assert.ok(err));
-}, TypeError, 'Invalid non-string/buffer chunk');
+}, /^TypeError: Invalid non-string\/buffer chunk$/);
 assert.doesNotThrow(() => {
   const m = new MyWritable().on('error', (e) => {
     assert.ok(e);