* Popen cleanup.
* Improve the test for response headers.
* Fix default headers of Httpx and H2 handlers, add tests.
* Check Location in redirect tests.
* Simplify bindaddress tests.
* Add download_latency to HttpxDownloadHandler, add tests.
* Remove explicit method="GET".
* Don't expect a specific value in test_download_with_maxsize_very_large_file
* Use a non-existent scheme in test_unsupported_scheme.
* Remporarily rollback the test_download_latency test check.
* Better check in test_download_latency.
* Improve logic for StreamCloseReason.CANCELLED.
* Switch TestEngineBase to mockserver.
* Split mockserver.py.
* Stop using WrappingFactory.
* Remove keyfile/certfile fields from TestHttpBase.
* Move webclient-only resources to test_webclient.py.
* Refactor MockServer.url().
* Migrate TestHttpBase to mockserver.
* Migrate TestSimpleHttpsBase to a separate mockserver.
* Rename TestHttpMockServerBase.
* Migrate TestHttpProxyBase to a separate mockserver.
* Refactor HTTP mockservers.
Andrey Rakhmatullin