Use default config trait in mock runtimes

[ggwpez]

Would be good if we start using the default config trait from #13454 to ensure that we covered most use-cases. For example in all mock.rs files on frame_system and maybe also pallet-balances.
Now we can still make some adjustments before more downstream projects start using it.
cc @juangirini

(This brings me to the idea that we could have a nightly or unstable module in frame-support, where we throw stuff in that has not been battle tested.)

[kianenigma]

Isn't this one item in paritytech/polkadot-sdk#171?

[juangirini]

It looks like a good candidate for a sub issue of the tracking one

[kianenigma]

Still seems like the list of pallets I have in #14453 :D

[ggwpez]

yea, duplicate.