Cache test update

* Added docblock to `_configCache()`
* Made assertions in `testNonFatalErrorsWithCacheDisable`. (To avoid "R" (Risky)  in phpunit.)
* Renamed `testNonFatalErrorsWithCachedisable` to `testNonFatalErrorsWithCacheDisable`.

There is *apparently* no way to tell PHPUnit there is no assertions in it :/ 
https://github.com/sebastianbergmann/phpunit-documentation/issues/171#issuecomment-38792136