Fix tests for PHP 8.3

[odan]

This PR should fix multiple compatibility issues with PHP 8.3 for test code only.

Example error message:

PHP Fatal error:  Declaration of Slim\Tests\Mocks\SmallChunksStream::close() 
must be compatible with Psr\Http\Message\StreamInterface::close(): void in 
/home/runner/work/Slim/Slim/tests/Mocks/SmallChunksStream.php on line 41

See here: https://github.com/slimphp/Slim/actions/runs/9737805348/job/26870413208

[coveralls]

coverage: 99.429% (+0.002%) from 99.427%
when pulling 9a07ee2 on odan:test-php-83
into b8e7b5c on slimphp:4.x.

[coveralls]

coverage: 99.427%. remained the same
when pulling 5eeaa19 on odan:test-php-83
into b8e7b5c on slimphp:4.x.

[coveralls]

coverage: 99.429% (+0.002%) from 99.427%
when pulling af2b5ac on odan:test-php-83
into b8e7b5c on slimphp:4.x.