Fix submodule checkout
Description
When the parent repo is cloned shallowly (aka in CI) the submodule is not automatically cloned shallowly, however the value of remote.origin.fetch is restricted to the branch it had been pointing at. Since the fetch is restricted only to the current/default branch, triggered pipelines from testdata branches fail in CI because it can't find the desired branch.
Except it wasn't failing because make set-testdata-branch didn't && chain the git commands. So the branch change failed and CI ran as normal on the original branch.
This fixes both of those problems.