tstest/natlab/vnet: stop agent dials from outliving their Server

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While a VM reboots, tests poll its agent roughly every 5s. Each
request that times out leaves behind a goroutine blocked in
Server.takeAgentConn, waiting for TTA to connect back. Several can
pile up before a request succeeds and the test ends, and the rest
wait forever.

Making takeAgentConn return at Server shutdown unblocks any goroutine
still waiting when the test ends.

In a full vmtest run, the two dials stranded by the reboot in
TestGokrazyUpdatesItselfToSameImage logged until the package exited
10 minutes later. With this change, "still waiting" log lines in the
same run drop from 267 to 72, and the longest wait goes from 10m10s
to 40s.

Updates #cleanup

Change-Id: Ib4c1d2ce8f2a7b73d3f0c1e6a9e5b2d8f4a1c7e3
Signed-off-by: Francois Marier <francois@tailscale.com>

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