sql/instancestorage: TestRefreshSession tries to use logic that is not copacetic

**Describe the problem**

When pointing TestRefreshSession to a secondary tenant, we get the following error:
```
sql_runner.go:322: error executing 'SELECT count(*) FROM system.sql_instances WHERE id = 1 AND session_id <> decode('010180bafc8a4b9cfa441db9fee7401e7791f6', 'hex')': pq: no healthy sql instances available for planning
```

Which is hardly surprising, given the test removes the rows just before.

It's unclear what the test is trying to do -- removing the rows in that table will block SQL planning, which will prevent further SQL.

**Expected behavior**

The test works equally well when pointed to a secondary tenant.

cc @dt

Jira issue: CRDB-30910

Epic CRDB-26687