ipn: close logger at the end of TestLocalLogLines

If any goroutine continues to use the logger in TestLocalLogLines
after the test finishes, the test panics.

The culprit for this was wireguard-go; the previous commit fixed that.
This commit adds suspenders: When the test is done, make logging calls
into no-ops.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
This commit is contained in:
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