Consolidate all `cardano-ledger-[era]-test` package

Every test package that is attached to each era no longer needs to be a separate package, since we have public `testlib` sub-libraries for exporting test functionality to downstream packages. This means test suites can migrate as well into the `cardano-ledger-[era]` package, which will reduce the release and maintenance burden and make it more organized.

The final goal with respect to tests is to:
* migrate all of the era specific tests into each era package
* abstract as much of era agnostic reusable test functionality into `cardano-ledger-core:testlib` and reuse it in all test suites
* switch all tests to use hspec, instead of tasty
* switch any left over tests from using hedgehog to using QuickCheck

This is a broad ticket that will require quite a bit of work and organization. It is an umbrella issue for the long term goal of organizing and improving our tests.

As far as this ticket is concerned, it is important to note that none of this applies to:
* Constraint based test framework
* most parts of `cardano-ledger-test` in general
* Byron era codebase.