Fix for Buildbot EUnit test failure

Buildbot isn't quite right in the head: I've no idea why I can
run this test 3000x without failure, but Buildbot fails

    bitcask_qc: merge2_test...
    =ERROR REPORT==== 5-May-2014::04:56:18 ===
    ** Generic server <0.2324.0> terminating
    ** Last message in was {'DOWN',#Ref<0.0.0.50927>,process,<0.2317.0>,normal}
    ** When Server state == {state,undefined,undefined}
    ** Reason for termination ==
    ** {{badmatch,{error,badarg}},
        [{bitcask_file,handle_info,2,[{file,"src/bitcask_file.erl"},{line,193}]},
         {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,604}]},
         {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}

Fix: Really, we're stopping, so change the pattern match to '_' so we
don't care.