Ensure `bundle update` runs succesfully in specs of multi-platform cache behavior

While implementing #3026, I noticed the `bundle update` run in the specs for multi-platform cache behavior was actually failing, causing the test to pass trivially (nothing was updated, so the cache wasn't affected). These changes ensure that the run actually succeeds. Also, it seems these specs were also (partially) testing the list of allowed platforms. I moved that part to some more focused tests.