The "more" cmd varies across Windows flavors, sometimes adding stray

The "more" cmd varies across Windows flavors, sometimes adding stray
newlines at the start or end.  Fiddle test_popen2 and popen2._test() to
tolerate this.  Also change all "assert"s in these tests to raise
explicit exceptions, so that python -O doesn't render them useless.
Also, in case of error, make the msg display the reprs of what we
wrote and what we read, so we can tell exactly why it's failing.