localrepo: Fix test to accommodate change in Git

In Git, the commit 757c6ee7a3 (builtin/push: call set_refspecs after
validating remote, 2024-07-11) fixed a regression WRT how git push
operates when an empty remote was provided. This also meant the error
that the user would receive changed. Modify our test to check for this
error specifically for the newer Git versions.