improve riak_test bash completion

Add missing riak_test options. Allow completion to work if riak_test is
invoked as ./riak_test or a similar pathname. Change the grep for "confirm"
to "confirm/0", which should be found in a test module export, so it
doesn't accidentally catch the plain word "confirm" in a comment in a
module that shouldn't be part of the testname completion. The
_get_comp_words_by_ref helper function is not available by default on OS X,
so add code to compensate for that case. Use a local variable to capture
the grep output, rather than overwriting a global variable.