Slow tests under a feature flag

[LesnyRumcajs]

Issue summary

It would be great to put slow tests under a dedicated feature flag, so a regular cargo test is more convenient to use during development. Currently, e.g. this test takes significant amount of time:

test test_download_location_of_proof_parameter_files_env has been running for over 60 seconds

Naturally, the CI should run both slow and fast tests. We could say that 1s is the cut-off line (any test running for longer is slow). While 1s is not a particularly long period, 100 tests of 1s would risk injuries during Forest development.

Perhaps there is a dedicated test framework for that?

Other information and links