Reorganize test utils

**Is your feature request related to a problem? Please describe.**

Many tests in lodestar, fork choice, etc depend on test utilities defined in `state-transition/test/util`.
There may be other instances of test utilities being imported across packages.

**Describe the solution you'd like**

Export test utilities in a separate package. This could include many/any utilities that help generate states or other consensus objects.

**Describe alternatives you've considered**

Leave as is.

**Additional context**

See https://github.com/ChainSafe/lodestar/blob/master/packages/fork-choice/test/perf/protoArray/computeDeltas.test.ts#L9 as an example