Reorganize test utils

[wemeetagain]

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