test: check complete error message

In test-stream-writable-change-default-encoding, use a regular
expression to match the complete error message.

PR-URL: #14264
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>

test/parallel/test-stream-writable-change-default-encoding.js

@@ -40,7 +40,7 @@ assert.throws(function changeDefaultEncodingToInvalidValue() {
   m.setDefaultEncoding({});
   m.write('bar');
   m.end();
-}, TypeError);
+}, /^TypeError: Unknown encoding: \[object Object\]$/);
 
 (function checkVairableCaseEncoding() {
   const m = new MyWritable(function(isBuffer, type, enc) {