logictest: add mixed-version tenant bootstrap test case

This commit enhances the testserver_upgrade_node logic test suite with
cases that verify that in a mixed-version cluster, tenant creation
bootstraps the same data in the tenant span regardless of which
executable is currently performing it. Until the upgrade is finalized,
the executables for the new release must behave exactly like those for
the old release.

Informs #94773.

Release note: None