fix #937: spawn tests to avoid unrelated messages

On slow systems the riak_kv_eleveldb_backend retry tests could fail due to
gen_event timeout messages from previous activities. Use a test fixture to
spawn these tests to ensure clean message queues.
Merge pull request #939 from basho/vinoski/fix-937

fix #937: spawn tests to avoid unrelated messages

Reviewed-by: reiddraper