test: flaky TestRUMXForwardedFor

From [CI logs](https://github.com/elastic/apm-server/actions/runs/27427590507/job/81069297292?pr=21157#step:6:276):
```
=== RUN TestRUMXForwardedFor
rum_test.go:73: []any{
map[string]any{
... // 2 identical entries
"agent.name.text": []any{string("rum-js")},
"agent.version": []any{string("5.5.0")},
- "client.geo.city_name": []any{string("dynamic")},
- "client.geo.continent_name": []any{string("Oceania")},
- "client.geo.country_iso_code": []any{string("AU")},
- "client.geo.country_name": []any{string("Australia")},
- "client.geo.location": []any{string("dynamic")},
- "client.geo.region_iso_code": []any{string("dynamic")},
- "client.geo.region_name": []any{string("dynamic")},
"client.ip": []any{string("220.244.41.16")},
"data_stream.dataset": []any{string("apm.internal")},
... // 26 identical entries
},
map[string]any{
... // 2 identical entries
"agent.name.text": []any{string("rum-js")},
"agent.version": []any{string("5.5.0")},
- "client.geo.city_name": []any{string("dynamic")},
- "client.geo.continent_name": []any{string("Oceania")},
- "client.geo.country_iso_code": []any{string("AU")},
- "client.geo.country_name": []any{string("Australia")},
- "client.geo.location": []any{string("dynamic")},
- "client.geo.region_iso_code": []any{string("dynamic")},
- "client.geo.region_name": []any{string("dynamic")},
"client.ip": []any{string("220.244.41.16")},
"data_stream.dataset": []any{string("apm.rum")},
... // 27 identical entries
},
}

Test failed. Run `make check-approvals` to verify the diff.

server.go:144: log file: /home/runner/work/apm-server/apm-server/systemtest/logs/TestRUMXForwardedFor/apm-server

```