TestMonitorMode skips by default, without the --monitor flag, and then
it previously ran forever. This adds an option --monitor-duration flag
that defaults to zero (run forever) but if non-zero bounds how long
the tests runs. This means you can then also use e.g. `go test
--cpuprofile` and capture a CPU/mem profile for a minute or two.
Updates #7621
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>