Split memcached_testapp into individual binaries per test suite

Currently all of the end-to-end tests in memcached is linked into memcached_testapp, and we invoke it with various --gtest_filter to allow cmake to run them all in parallel (to speed up test execution). 

By specifying --gtest_filter we may (and have multiple times already) end up in the situation that some tests won't be run as they don't match any of the filters we specify.

Given that we want to be able to run the test batches in parallel we should rather change CMakeLists.txt to build individual executables per logical test batch so that it may be started without --gtest_filter.