Every tool derired from deqp-runner uses the same pattern to deploy the
results file in RunnerCommandResult to be mocked for integration tests.

Let's implement a constructor-like method `from_output` to simplify this
and make it easier to add the same logic for the `expectations.json`
file.

Signed-off-by: Guilherme Gallo <guilherme.gallo@collabora.com>