Fix parallelism bug in exit processing (#1110)

* Fix parallelism bug in exit processing

Also:

* Remove parallelism for all other operations except deposit merkle proofs
* Improve exit tests
* Fix broken attestation test

Closes #1090

* Allow for generating block/pre/post states from some unit tests (#1123)

* Add post-state checks, comments

* Add state_transition_vectors crate

* Integrate new testing crate with CI

* Add readme

* Add additional valid tests

* Remove ExitTests (they were moved to new crate)

* Small test fixes

* Delete incorrect saturating_sub in slash_validator

And clean-up the balance increase/decrease functions to look more like the spec.

Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/beacon_chain/tests/store_tests.rs

@@ -1,4 +1,4 @@
-// #![cfg(not(debug_assertions))]
+#![cfg(not(debug_assertions))]
 
 #[macro_use]
 extern crate lazy_static;