dev-docs: clarify which scripts/setups to use for which usecases

Update the [Manual Testing instructions](https://github.com/elastic/apm-server/blob/main/dev_docs/TESTING.md#manual-testing) to clarify that the `Tilt` setup brings up the apm-server binary, whereas using `systemtest/cmd/runapm` brings up an Elastic Agent and Fleet managed apm-server.
The [Testing Stack monitoring](https://github.com/elastic/apm-server/blob/main/dev_docs/TESTING.md#testing-stack-monitoring) instructions should be adapted to cover enabling stack monitoring for both ways of running the stack and making it clear that the `testing/stack-monitoring.sh` script is not compatible with running the `tilt` setup.