ensured test instance's `tiddlywebconfig.py` is loaded

thus resolving the config issue mentioned in the previous commit
(56eef3ec19bd2203b5c232bbbe87a65cf2bdb975)

note that this required properly calculating the respective cookie as
`CONFIG['secret']` is no longer predictable
moved static initialization into config

now possible thanks to proper test instance (cf. previous commits;
56eef3ec19bd2203b5c232bbbe87a65cf2bdb975 and
bdfc20db812b0ff3182c58e0ed2fab8da96944a1)