Found while working on #110001.

Describe the problem

This test misbehaves when pointed to a secondary tenant; that is, the following sequence fails:

   client, _ ≔ s.GetAuthenticatedHTTPClient(false, serverutils.SingleTenantSession)
   resp, _ ≔ client.Post(s.AdminURL().WithPath("/logout").String(), "", nil)
   require.Equal(t, 200, resp.StatusCode)

The error is "expected 200, got 405" . It's strange and unexpected that /logout on a secondary tenant would return 405.

To Reproduce

Change the test as follows:

 srv ≔ serverutils.StartServerOnly(t, base.TestServerArgs{})
 s := srv.ApplicationLayer()

Epic CRDB-38970

Jira issue: CRDB-31212