tstest/integration: add start of integration tests for incremental map updates

This adds a new integration test with two nodes where the first gets a
incremental MapResponse (with only PeersRemoved set) saying that the
second node disappeared.

This extends the testcontrol package to support sending raw
MapResponses to nodes.

Updates #1909

Change-Id: Iea0c25c19cf0d72b52dba5a46d01b5cc87b9b39d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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