refactored test setup's app initialization

it appears the use of `instance_config` for this (in
bdfc20db812b0ff3182c58e0ed2fab8da96944a1) was the result of insufficient
understanding of the config initialization process

not that I fully grok it now, but this seems more correct - it allows
for a dev instance's `tiddlywebconfig.py` to be present in the repo root
without interfering with the tests