Add riak_test to exercise riak-admin shell script

comments pulled from tests/riak_admin_console_tests.erl

```
%% This test passes params to the riak-admin shell script on to intercepts
%% that either return ?PASS or ?FAIL (which print out "pass" or "fail" to
%% the console). If an unexpected input is received in Erlang, ?FAIL is
%% returned. This test should (will?) make sure we don't implement
%% any unportable shell code. For example, `riak-repl cascades foo`
%% didn't work on Ubuntu due to an invalid call to shift. Since this test
%% will be run on giddyup and hence many platforms, we should be able
%% to catch these types of bugs earlier.
%% See also: replication2_console_tests.erl for a more detailed
%% description. 
```

I'll submit to giddyup upon merge.