ipn/ipnlocal: fix missing controlknobs.Knobs plumbing

I missed connecting some controlknobs.Knobs pieces in 4e91cf20a8
resulting in that breaking control knobs entirely.

Whoops.

The fix in ipn/ipnlocal (where it makes a new controlclient) but to
atone, I also added integration tests. Those integration tests use
a new "tailscale debug control-knobs" which by itself might be useful
for future debugging.

Updates #9351

Change-Id: Id9c89c8637746d879d5da67b9ac4e0d2367a3f0d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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