k6runner: send logs even if metrics are malformed

Another step towards https://github.com/grafana/synthetic-monitoring-app/issues/835.

When https://github.com/grafana/sm-k6-runner/pull/208 is merged, there is a theoretical possibility of the logs and/or metrics returned by the runner to be malformed, for example is k6 is killed while it is writing its output.

This PR adds tests for that case, ensuring that:

Receiving borked metrics triggers an error
Receiving borked metrics still causes logs to be sent, and the execution to be marked as failure.

grafana / synthetic-monitoring-agent

k6runner: send logs even if metrics are malformed #742