[9.x] Add streamed content test assertion

[ziadoz]

This PR of mine was merged but then reverted because the tests were broken (sorry!). This PR fixes the tests.

❯ ./bin/test.sh -- --filter=TestResponseTest
Ensuring docker is running
Ensuring services are running
laravel-framework_redis_1 is up-to-date
laravel-framework_mysql_1 is up-to-date
laravel-framework_memcached_1 is up-to-date
laravel-framework_dynamodb_1 is up-to-date
Running tests
PHPUnit 9.5.27 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.26
Configuration: /data/phpunit.xml.dist

...............................................................  63 / 138 ( 45%)
............................................................... 126 / 138 ( 91%)
............                                                    138 / 138 (100%)

Time: 00:00.236, Memory: 100.50 MB

OK (138 tests, 395 assertions)