Implement `Default` for `Market`

[maltekliemann]

This is useful so not every pallet has to implement its own version of market_mock.

[sea212]

Guess it will be tricky to define a Default as it depends on the runtimes configuration of the prediction markets pallet. I assume it makes sense to define it in the test environment, as we can utilize the mock values within zrml-primitives in that case.

[maltekliemann]

Was probably a bad idea.