net/http: in the IdleConnStrsForTesting_h2 helper, omit conns that cannot be reused

In #59155, we observed that the IdleConnStrsForTesting_h2 helper
function sometimes reported extra connections after a
"client conn not usable" failure and retry. It turns out that that
state corresponds exactly to the
http2clientConnIdleState.canTakeNewRequest field, so (with a bit of
extra nethttpomithttp2 plumbing) we can use that field in the helper
to filter out the unusable connections.

Fixes #59155.

Change-Id: Ief6283c9c8c5ec47dd9f378beb0ddf720832484e
Reviewed-on: https://go-review.googlesource.com/c/go/+/477856
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

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