test: Clear test-output output file before running Emacs tests

Most Emacs tests end with a call to (test-output), which saves the
buffer to a filed called OUTPUT.  Previously, if the test code failed
with an exception before this call, the test framework would then
compare against the OUTPUT file from the last Emacs test, resulting in
confusing diffs.

This requires one tweak to an emacs test that made two calls to
test_emacs and expected an OUTPUT file from the first call.  We simply
reverse the order of the test_emacs calls.