tstest/integration: prefix logs with logid

The VM test has two tailscaled instances running and interleaves the
logs. Without a prefix it is impossible to figure out what is going on.

It might be even better to include the [ABCD] node prefix here as well.
Unfortunately lots of interesting logs happen before tailscaled has a
node key, so it wouldn't be a replacement for a short ID.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
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