Fix test bot regressions and add regression specs

- test_formulae: use [] instead of fetch for @downloaded_artifacts so new
  SHAs use the hash default (empty array) and avoid KeyError
- junit: require rexml/document, xmldecl, cdata in #initialize so REXML
  is loaded before use (fixes uninitialized constant on Linux CI)
- test_cleanup: pass HOMEBREW_REPOSITORY.to_s to checkout_branch_if_needed,
  reset_if_needed, clean_if_needed (fixes Pathname type error when tap set)
- Add test_formulae_spec to prevent KeyError regression for new SHA access
- Add junit_spec to prevent REXML NameError regression
- Add test_cleanup_spec to prevent Pathname regression