all: make more tests pass/skip in airplane mode

Updates tailscale/corp#19786

Change-Id: Iedc6730fe91c627b556bff5325bdbaf7bf79d8e6
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