I was running all tests while preparing a recent stable release, and
this was failing because my computer is connected to a fairly large
tailnet.
```
--- FAIL: TestGetRouteTable (0.01s)
routetable_linux_test.go:32: expected at least one default route;
...
```
```
$ ip route show table 52 | wc -l
1051
```
Updates #cleanup
Signed-off-by: Anton Tolchanov <anton@tailscale.com>