net/routetable: increase route limit used by the test

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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>

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