Mock epoch index and randomness in SproofBuilder (for testing) (#1594)

* minimal babe randomness in sproof builder

* fix indexing

* fix index and clean comment

* use current para block for epoch and randomness

* epoch is para block randomness is relay block

* fix

* make more configurable but keep defaults

* fix export

* fix

* flatten

* better field names

* Update primitives/parachain-inherent/src/mock.rs

* Update primitives/parachain-inherent/src/mock.rs

* FMT

* FIX

* fmt

* Update primitives/parachain-inherent/src/mock.rs

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Bastian Köcher <info@kchr.de>