kube: handle goaway test fixture conns concurrently

[jakealti]

Follow-up to #66672. The fake goawayServer in forwarder_test.go calls handleConn inline, so concurrent clients in TestGOAWAYHandling_Concurrent are serialized. Under -race + slow CI, the 50-deep backlog can outlast the per-request 10s context, leaving the httptest handler stuck in TLS handshake and forwarderServer.Close() waiting on it until Go's 10-minute test timeout fires.

Example: https://github.com/gravitational/teleport/actions/runs/26193145084/job/77066278419?pr=66930

Fix: spawn a goroutine per accepted conn. No production code touched.

Stress: stress -p 8 -test.cpu=1 -race for ~4 min — 1,317 runs, 0 failures.

[espadolini]

Do we want a way to synchronously wait for all the handling goroutines to exit? It's something that we indirectly get from synctest but TestGOAWAYHandling and TestGOAWAYHandling_Concurrent use real listeners and don't use synctest.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prevent Close from returning before accepted conn handlers start

A connection accepted just before Close() takes g.mu can be appended and launched after the close loop unlocks, because Serve() does append+wg.Go under the same mutex but Close() releases the mutex before wg.Wait(). In that interleaving, Close() can miss closing that conn and return without waiting for its handler, leaving a goroutine blocked in TLS/frame reads and reintroducing teardown flakiness under concurrent tests.

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[jakealti]

Minor but I addressed in 8e73d4d