net/http: eliminate arbitrary timeouts in TestServerRequestContextCancel_ConnClose

net/http: eliminate arbitrary timeouts in TestServerRequestContextCancel_ConnClose

These timeouts are empirically sometimes (but rarely) too short on
slower builders, and at any rate if this test fails “for real” we'll
want a goroutine dump in order to debug it anyway. A goroutine dump is
exactly what we get if we let the test time out on its own.

Fixes #52414.

Change-Id: Id2dd3839977bd8a41f296d67d1cccbf068fd73f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/400816
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>