tests: add tests to test-unit-name

add tests for:
  - unit_instance_is_valid
  - unit_prefix_is_valid
  - unit_name_change_suffix
  - unit_name_build
  - unit_name_is_instance
  - build_subslice
  - unit_name_to_instance
  - unit_name_escape
tests: add test-ratelimit
build-sys: do not run coverage if build failed