t3200: test --set-upstream-to with bogus refs

t3200: test --set-upstream-to with bogus refs

These tests pass with the current code, but let's make sure
we don't accidentally break the behavior in the future.

Note that our tests expect failure when we try to set the
upstream to or from a missing branch. Technically we are
just munging config here, so we do not need the refs to
exist. But seeing that they do exist is a good check that
the user has not made a typo.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>