refactor: replace wiremock with own test harness

Reasons:
- there are too many layers of types that transfer/modify things
- we gonna need a test harness that supports TLS/HTTPs
- we need kinda precise control over which protocols are used to know
  what we're actually testing here
- I wanna use something simple, i.e. not a full-blown "we launch a
  server in the background" magic thing like `httpmock`

Author: crepererum