[release-branch.go1.15] net/http/fgci: skip flaky test

[release-branch.go1.15] net/http/fgci: skip flaky test

A test introduced in the security release is flaky due to a pre-existing
issue that does not qualify for backport itself.

Updates #41167
Fixes #41193

Change-Id: Ie6014e0796c1baee7b077881b5a799f9947fc9c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/252717
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>