wgengine/filter: fix, test NewAllowAllForTest

I probably broke it when SCTP support was added but nothing apparently
ever used NewAllowAllForTest so it wasn't noticed when it broke.

Change-Id: Ib5a405be233d53cb7fcc61d493ae7aa2d1d590a2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f