cardano-testnet: Towards removing the TestnetRuntime data type

[MarcFontaine]

The current scheme for tests is:

test1 = do
  testnetRuntime <- testnet ...
  test1Core testnetRunTime
test2 = do
  testnetRuntime <- testnet ...
  test2Core testnetRuntime

testnet starts a cluster and returns a TestnetRuntime which is passed to the actual test. Problems:

• TestnetRuntime is just a record/product type. A sum type would be needed to support different testnet setups.
• TestnetRuntime is growing bigger and bigger.
• The same TestnetRuntime is passed to every test, even though each test uses only parts of the data.
• It is not easy to track which data is used where.
• There are cases where tests by-pass TestnetRuntime and just read data from the file system.

This PR is WIP towards removing TestnetRuntime. The idea is tests are called directly inside testnet.