[BUGFIX] Tests: MemcacheBackendTest throws warnings

setUp() of memcache cache backend tests checks with fsockopen() if
a connect to memcache is possible to skip the tests if memcache is
not available. If fsockopen fails it throws a warning, this should
be suppressed.

Change-Id: I8590d20201e76f0768e5beb0c9b549b2b48933ec
Resolves: #56885
Releases: 6.2
Reviewed-on: https://review.typo3.org/28357
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring