Fix meck related unit test problems

Meck now needs a couple of flags in meck:new for some of our tests:
* non_strict so we can create a fresh non-existent module
* no_link so it doesn't link to the processes created with a meck
* module. Bad when we kill things on purpose.

Trying non_strict meck flag in riak_core_stat_cache.
The build failed with an undefined_module error setting up this text. I
can not get that to happen locally, but this flag has fixed similar
problems in other tests.