* Alerting: Fix Test Receivers when settings are non-strings
As part of the Alerting extraction, we want to make sure we don't have circular depedencies. As such, I had to move `PostableGrafanaReceiver` to a new struct in `grafana/alerting` called `GrafanaReceiver`.
`PostableGrafanaReceiver` has an attribute called `Settings` that uses a Grafana-propietary struct called `RawMessage`, this struct shadows `json.RawMessage`.
When I created `GrafanaReceiver`, I turned settings into a `map[string]string` thinking all settings would end up as strings. This was a mistake, and this test proves that it doesn't work, and breaks the API.
0bfe150928