We have 100+ tests of multitenant migrations that don't pass "protocol" to donorStartMigration (they accept the default value, "multitenant migrations"). Let's reuse these tests, but modify them to pass protocol: "slice merge" when featureFlagSliceMerge is enabled.

Since nearly all the tests call donorStartMigration via the TenantMigrationUtil helper, we can make the change in just a few locations.