Add hscontrol/types/testcapture, mirroring the type that the tscap tool
emits. The package has no tailscale.com/tailcfg import so it stays
dependency-free; wire-format Tailscale data is stored as
json.RawMessage and consumers unmarshal into the typed shape they need.
Drop the four private *TestFile structs from the policy v2 compat tests
and read directly into testcapture.Capture. Switch globs to lowercase
(acl-/grant-/routes-/ssh-) to match the new tscap output filenames.
Updates #3157
Updates #3169
Kristoffer Dalby
e30de58627