There's a 50ms threshold in net/http.Transport that this test
sometimes hitting on slower devices. That was unrelated to what this
test was trying to test. So instead just t.Skip on RoundTrip errors
unless the failure was quick (under 25ms), in which case the error
must've been about something else. Our fast machines should catch
regressions there.

Fixes #25366

Change-Id: Ibe8e2716a5b68558b57d0b8b5c46f38e46a2cba2
Reviewed-on: https://go-review.googlesource.com/125555
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>