Refactor testsetup handling

[nickrobinson251]

• Populate testsetup field with Dict mapping name to code, so we can rely solely on testsetup field and lookup code directly.
  • Hopefully makes adding Have @testsetup automatically available to @testitem defined in the same file #60 easier (we can just add them to the testsetup field, and leave the setup field untouched as the field that stores the explicltly stated setups=[...])
• Factor the code for running the testsetups for a testitem out into its own function
  • Hopefully makes Time spent evaluating test-setups shouldn't be included in test-item timeout #140 easier (we can call runtestsetups to ensure all the setup modules are created, perhaps with its own timer, before running runtestitem against the timer)