Tests: fixed CacheStorage's sliding tests [Closing #958]

These tests failed on Travis-ci.org accidentally. It's because sliding interval was so tight.

I verified, that sliding functionality is working well in both cases (MemcachedStorage, FileStorage). The expiration of data is properly prolonged on every reading.

But there is a potential weak place. Cache::completeDependencies() converts expiration time to relative amount of seconds. When user sets expiration at time() + 2, until Cache::completeDependencies() runs, time() value increase. So, expiration time is decreased by 1 second in consequence. I guess, this is a reason why tests failed on Travis (virtual server, time jitter, I/O rush).

I hope, this feature needn't be fixed, Cache is not permanent data storage. And nobody complain.
Merge pull request #959 from milo/fix/tests/memcache-sliding

Tests: fixed Memcached.sliding.phpt [Closing #958]