erlc: fix comment

test_compile/3 is used by eunit and qc, but the comment was only
referring to 'rebar eunit'.
Fix #220 (Reported-by: Joseph Norton)

When running 'rebar qc' or 'rebar eunit', we were erroneously fetching
erl_opts more than once.
Merge pull request #234 from tuncer/fix-220

Fix #220 (Reported-by: Joseph Norton)