security: HTTP cert tests fail with secondary tenants

**Describe the problem**

The following tests fail when pointed to a secondary tenant:

- TestUseCerts
- TestUseSplitCACerts

Example error:
```
    certs_test.go:452: Expected OK, got "401 Unauthorized" with body: a valid authentication cookie is required
```

**Expected behavior**

The tests should work


Epic CRDB-38970
Jira issue: CRDB-30954