chore: fix test assertions

[kvcache]

integration test assertions are currently missing all details in
the most expected case: When they got an error back.

This change makes tests output more helpful & verbose failures.
Replaces assertion messages like:

8) Momento\Tests\Cache\CacheClientTest::testListPushFrontFetchHappyPath
Failed asserting that null is not null.

/Users/kenny/gits/client-sdk-php/tests/Cache/CacheClientTest.php:427

9) Momento\Tests\Cache\CacheClientTest::testListPushFront_TruncateList
Failed asserting that null is not null.

/Users/kenny/gits/client-sdk-php/tests/Cache/CacheClientTest.php:480

With something a little more like:

35) Momento\Tests\Cache\CacheClientTest::testListPushBack_NoRefreshTtl
Failed asserting that Momento\Cache\CacheOperationTypes\CacheListFetchResponseError Object &00000000000009a20000000000000000 (
    'baseType' => 'Momento\Cache\CacheOperationTypes\CacheListFetchResponse'
    'innerException' => Momento\Cache\Errors\ServerUnavailableError Object &00000000000009a70000000000000000 (
        'message' => 'Resource unavailable'
        'string' => ''
        'code' => 14
        'file' => '/Users/kenny/gits/client-sdk-php/src/Utilities/_ErrorConverter.php'
        'line' => 46
        'previous' => null
        'errorCode' => Momento\Cache\Errors\MomentoErrorCode Object &00000000000006690000000000000000 (
            'name' => 'SERVER_UNAVAILABLE'
        )
        'messageWrapper' => 'The server was unable to handle the request; consider retrying.  If the error persists, please contact us at support@momentohq.com'
    )
    'errorCode' => Momento\Cache\Errors\MomentoErrorCode Object &00000000000006690000000000000000
    'message' => 'The server was unable to handle the request; consider retrying.  If the error persists, please contact us at support@momentohq.com: Resource unavailable'
) is null.

/Users/kenny/gits/client-sdk-php/tests/Cache/CacheClientTest.php:619

36) Momento\Tests\Cache\CacheClientTest::testListPushFront_NoRefreshTtl
Failed asserting that Momento\Cache\CacheOperationTypes\CacheListFetchResponseError Object &00000000000009b80000000000000000 (
    'baseType' => 'Momento\Cache\CacheOperationTypes\CacheListFetchResponse'
    'innerException' => Momento\Cache\Errors\ServerUnavailableError Object &00000000000009bd0000000000000000 (
        'message' => 'Resource unavailable'
        'string' => ''
        'code' => 14
        'file' => '/Users/kenny/gits/client-sdk-php/src/Utilities/_ErrorConverter.php'
        'line' => 46
        'previous' => null
        'errorCode' => Momento\Cache\Errors\MomentoErrorCode Object &00000000000006690000000000000000 (
            'name' => 'SERVER_UNAVAILABLE'
        )
        'messageWrapper' => 'The server was unable to handle the request; consider retrying.  If the error persists, please contact us at support@momentohq.com'
    )
    'errorCode' => Momento\Cache\Errors\MomentoErrorCode Object &00000000000006690000000000000000
    'message' => 'The server was unable to handle the request; consider retrying.  If the error persists, please contact us at support@momentohq.com: Resource unavailable'
) is null.

/Users/kenny/gits/client-sdk-php/tests/Cache/CacheClientTest.php:518

We can certainly do more to make these messages better, but at least
this shows errors that come back from the server