Allow cache xcache tests to run under cli SAPI.

We test the caching/storage features of xache here to the opcode features. Opcode features are only available when running in web context.
Fix xcache tests expecting false instead of null.
Document second Cache::write syntax.
Pass only required set of options to Cache::delete strategies.
Adding test for not existent key to memory adapter.
Document no expiry for memory cache adapter.
Adding note about possible future deprecation of Cache::delete strategies.
Reduce line count for cache conditions.
Extracting integration tests from cache unit test.
Whitespace.
Fixing cache strategies with multi-keys.

When using the (undocumented) mulit-keys feature strategies were applied to the array
result and not each single result. This lead to unexpected behavior i.e. when using the
searialize strategy.

Adding tests.