net/http: fix flaky TestServerValidatesHostHeader
net/http: fix flaky TestServerValidatesHostHeader

Passes with go test -race -count=1000 -name=TestServerValidatesHostHeader now
without hanging.

Fixes #13950

Change-Id: I41c3a555c642595c95c8c52f19a05a4c68e67630
Reviewed-on: https://go-review.googlesource.com/18660
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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