http2: use testing/synctest

Replace ad-hoc pre-synctest synchronization with testing/synctest.

Many of the http2 package tests use a custom-built fake time
implementation and a idleness-detection mechanism based on parsing
goroutine stack dumps. Experience with this approach to testing
eventually led to the development of the testing/synctest package.
Switch over to testing/synctest.

The synctest package became available as an experiment in Go 1.24
(only when GOEXPERIMENT=synctest is set), and was fully released
with some API changes in Go 1.25.

- Use the released synctest API on Go 1.25.

- Use the experimental API (synctest.Run) on Go 1.24 when
  GOEXPERIMENT=synctest is set. (Note that we set this on trybots.)

- Skip tests on Go 1.24 when GOEXPERIMENT=synctest is not set.

The x/net module requires go1.24, so older versions
can be disregarded.

Change-Id: Ifc13bfdd9bdada6c016730a78bd5972a5193ee30
Reviewed-on: https://go-review.googlesource.com/c/net/+/700996
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>