ccl/multiregion,regionlatency: unit tests are partially incorrect because cross-region latencies are not properly simulated with secondary tenants

This shares the same cause as #109867.

**Describe the problem**

The tests in the ccl/multiregion package depend on the `regionlatency` package to simulate cross-region latencies in a TestCluster. However, this is not done properly for secondary tenants.

This is because `regionlatency` injects the latencies only between the KV nodes, but not between the SQL servers.
Consider the connectivity diagram:

```
SQL <---(a)---> SQL
^ ^
| |
(b) (b)
| |
v v
KV <---(c)---> KV
```

The simulated latencies are injected for the connections of type (c), not in type (a) as they should.

**Expected behavior**

In the diagram above (a) and (c) should both use extra latencies.

Epic: CRDB-26687

Jira issue: CRDB-31126

Start in the ccl/multiregion tests and the regionlatency package, then trace how TestCluster connectivity is configured for secondary tenants. Compare the SQL-server and KV-node paths shown in the issue and run the affected unit tests. Done means both SQL and KV cross-region connections use the extra simulated latency.

Written by the indexing model from the issue text.