net/http: standardize HTTP/1 and HTTP/2 test paths

[neild]

The handling of HTTP/1 and HTTP/2 in net/http tests is ad hoc and inconsistent. We've got a number of pre-HTTP/2 tests that are fairly tied to the protocol version (writing HTTP/1 request bytes directly to a socket, for example), but there are tests which could trivially run on both protocol versions but don't. The tests which do run on both versions do so by manually setting up paths for both.

We should clean all this up: Run tests on all appropriate versions, using a standardized helper. This will also make it easier to test HTTP/3 if/when we support it. It'd also let us run other variations easily, such as HTTPS/1.

(I've already got CL 438137 to do this, filing an issue for bookkeeping/future reference.)

[gopherbot]

Change https://go.dev/cl/438137 mentions this issue: net/http: refactor tests to run most in HTTP/1 and HTTP/2 modes