runtest succeeds on non-existent directories

[vbgl]

When a scenario for easycrypt runtest lists a non-existent directory as okdirs, this results in a success.

I think that this behavior can lead to a false sense of confidence (“checks succeed!”) when the configuration is broken.

For instance, trying to run a non-existing scenario (easycrypt runtest config abcd) produces a failure (“runtest: error: unknown scenario: abcd”). I believe it would be more consistent to also fail when the okdirs lists incorrect paths.