net/http: use cancellation instead of a timeout in TestTransportProxyHTTPSConnectTimeout

The use of a timeout in this test caused it to be flaky: if the
timeout occurred before the connection was attempted, then the Accept
call on the Listener could hang indefinitely, and its goroutine would
not exit until that Listener was closed. That caused the test to fail.

A longer timeout would make the test less flaky, but it would become
even slower and would still be sensitive to timing.

Instead, replace the timeout with an explicit Context cancellation
after the CONNECT request has been read. That not only ensures that
the cancellation occurs at the appropriate point, but also makes the
test much faster: a test run with -count=1000 now executes in less
than 2s on my machine, whereas before it took upwards of 50s.

Fixes #36082
Updates #28012

Change-Id: I00c20d87365fd3d257774422f39d2acc8791febd
Reviewed-on: https://go-review.googlesource.com/c/go/+/210857


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>