Cleanup Julia Testsuite Modules

[FlorianFranzen]

This issue tracks open tasks from the CI rewrite and custom test-runtime integration work:

• Merge AdapterTests.jl and HostAPITests.jl (Fixed in 4298cec)
• Move Julia modules to separate folder (Fixed in 4298cec)
• AdapterFixture:
  • Use CmdList instead of StringList for input (Fixed in 60eb4f2)
  • Evaluate use of quote vs. current do scope (Fixed by adding CmdList)
  • Cache substrate-adapter results in fixtures
• Better summary of broken and skipped tests
• Move to Julia reference implementation for (some) tests
  • Reduce dependence on substrate-adapter
  • Close in on generating fixture from specification
• Refactor testsuite-fixture interface
  • Filter out sub-folders that do not include include.jl file
  • Avoid having include.jl iterate over all implementations on include(...)
  • Clean up run_fixture(), run(builder, ...) and execute(...)

Progress is documented through edits, items are removed once merged into master.