There are a number of tests that currently xfail in this suite.
The intention is to fix the issues one by one, or in cases where
the issue is a WONTFIX, change the test and/or indicate as such
in the xfail reason.

There is still a need for a couple more tests in the websocket
section, and probably could do with some more type-oriented
negative tests (e.g. testing what happens if you send a message
with the wrong type in the event values, like with 'status')

Fixes test_http:test_response_body_can_be_missing()

Fixes test_http:test_response_headers_can_be_missing()

Fixes test_websocket :
 test_asgi_version_is_present_in_websocket_scope() and
 test_websocket_spec_version_is_missing_or_correct()

Fixes test_lifespan:test_lifespan_not_supported_is_allowed()

This fix was somewhat challenging to achieve without breaking
other things or backward compatibility issues.

In the end, I think the most elegant solution is to adopt
more Pythonic exceptions in the TestClient, as demonstrated in
the commit. However, since this is a less trivial change, I
have only used TestClientError in the lifespan related code,
 leaving all other code using raw Exceptions still.

I think TestClientError (or possibly even multiple exception classes
for different purposes) should actually be adopted throughout
all the code, albeit with the caveat that it might cause some issues
if anyone is doing really rigid exception assertions in their test
code. For most cases (including my own code that is already using
async_asgi_testclient) I believe the average
 'with pytest.raises(Exception)' assertions should still work,
 since TestClientError is a sub-class of Exception.

A side-benefit of the TestClientError is it would allow access to
the original Message structure, instead of having to decode a string
repr of the event message.

Removed test for contamination between calls with a custom scope.
Probably deserves documentation because it could be a gotcha for people,
but it doesn't deserve to be in this test suite.

Had a few issues with formatting and the black version, this commit cleans that up.

Re: https://github.com/vinissimus/async-asgi-testclient/pull/50/files#r785889284

The asserts where we access the log message in the test is a bit rough, it could probably be refactored to be a little cleaner.