net/http: fix some test flakes caused by test refactoring

net/http: fix some test flakes caused by test refactoring

Skip TestTransportPersistConnLeakShortBody in HTTP/2 mode;
it's flaky and was previously HTTP/1-only.

Don't run TestTransportEventTrace and TestTransportIgnores408
in parallel.

Change-Id: I76bc540fac9317185ef7d414c9deafb35bc926b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/442495
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>