Add PARALLEL_TESTNETS flag

[carbolymer]

Description

This PR adds PARALLEL_TESTNETS environment flag which controls concurrent execution of testnets. By default concurrent execution is disabled.

This is also a workaround for broken TASTY_PATTERN which does not seem to work with sequentialTestGroup. To run a single test with a matching pattern, execute:

PARALLEL_TESTNETS=1 TASTY_PATTERN='/leadership-schedule/' cabal test cardano-testnet-test

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Runnings tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.

[smelc]

Confirm it fixes the tasty pattern with PARALLEL_TESTNETS=1 and also the change to cardano-testnet-test.hs is sound 👍