Move test server to a separately importable module

[florimondmanca]

Refs #650, prompted by #985 (comment)

Being able to import the test Server is required* for annotating tests.

*At least, it's required with our current test setup. Dunno if we might want to consider tweaking our mypy configuration with --check-untyped-defs, so that the inside of test functions is type checked even though their signature is not annotated. In general it's really the inside of tests (calls to HTTPX) that we want to make sure provide a good typing experience — type hinting the test function signatures doesn't provide a huge gain (but on the contrary a non-negligible amount of work is required if we did want to annotate them all).

So, marking this as a draft for now…

[florimondmanca]

Closing as I think #991 might be a better approach for us - one that will not even require adding server: Server annotations everywhere just to have mypy type check the inside of test functions.