storagemgr: Fix reftable tests which are broken with Git 2.37

In 8b958b18 (featureflag: Default enable Git v2.47, 2024-10-18), we made Git 2.47 the default Git version to be used. The new Git version adds support for compacting reftables, even when there are other tables locked. Some of our tests need to be amended to incorporate this.