don't write anything to stdout or stderr
Removed some superfluous print statements which were presumably added
for debugging.
Capture the stderr of rpmbuild and createrepo_c, so that we don't see it
normally. If the command fails, the exception message will include the
output from the command, which makes it easier to figure out what went
wrong if they do fail.
The end result is that rpmfluff itself, as well as its test suite, can
run without producing any spew.