Rename overload test module to avoid shielding the same name in sasl app

[shino]

There is a test module with name overload which is included in escriptized riak_test. It shields the module with the same name in sasl app, then riak_test for riak_cs output error logs.

2015-01-20 15:38:14.890 [error] <0.152.0> Supervisor sasl_safe_sup had child overload started with overload:start_link() at undefined exit with reason {'EXIT',{undef,[{overload,start_link,[],[]},{supervisor,do_start_child,2,[{file,"supervisor.erl"},{line,310}]},{supervisor,start_children,3,[{file,"supervisor.erl"},{line,293}]},{supervisor,init_children,2,[{file,"supervisor.erl"},{line,259}]},{gen_server,init_it,6,[{file,"gen_server.erl"},{line,304}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}} in context start_error

This PR renames the test module.