remove default threshold in test

[AdriMarteau]

Setting default threshold like 1 for accuracy test can have some limitation or risks.

On the risk side, a model with 100% accuracy is most likely a spurious one and the test would pass.
Also, this make it less transparent as to the parameters reported when executing the test like in Suite.run.

Removing the default value would also raise an error when defining the test and force the developper to better think about a proper level.

[henchaves]

Hey @kevinmessiaen and @mattbit, what is your opinion on that?