net/http: remove arbitrary timeouts from TestIdentityResponse and TestTLSHandshakeTimeout

These hard-coded timeouts make the tests flaky on slow builders (such
as solaris-amd64-oraclerel), and make test failures harder to diagnose
anyway (by replacing dumps of the stuck goroutine stacks with failure
messages that do not describe the stuck goroutines). Eliminate them
and simplify the tests.

Fixes #37327
Fixes #38112

Change-Id: Id40febe349d134ef53c702e36199bfbf2b6468ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/225977


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