Add failing test catching defunct launch.sh processses.

Reproduces issue #2836, i.e.:

- during initialisation:

```
$ ps auxf
[...]
root      1380  0.4  1.6 879660 60732 ?        Ssl  Mar13   5:54 /usr/bin/docker daemon -H fd:// -H unix:///
root      1664  0.0  0.4 502116 18540 ?        Ssl  Mar13   0:06  \_ docker-containerd -l /var/run/docker/li
root     12615  0.0  0.1 200496  5424 ?        Sl   14:38   0:00      \_ docker-containerd-shim c5637e5bbdcb
root     12629  6.2  1.6 296400 62312 ?        Ssl  14:38   0:00      |   \_ /home/weave/weaver --port=6783
root     12910  0.0  0.0   1524    68 ?        S    14:38   0:00      |       \_ /bin/sh /home/weave/launch.
root     13002  0.0  0.0   1772  1232 ?        S    14:38   0:00      |           \_ /bin/sh /home/weave/wea
root     13027  0.0  0.0   1772   320 ?        S    14:38   0:00      |               \_ /bin/sh /home/weave
root     13028  0.0  0.0  14656  2708 ?        S    14:38   0:00      |                   \_ curl -o /tmp/we
[...]
```

- after initialisation:

```
$ ps auxf
[...]
root      1380  0.4  1.6 879660 60732 ?        Ssl  Mar13   5:54 /usr/bin/docker daemon -H fd:// -H unix:///
root      1664  0.0  0.4 502116 18540 ?        Ssl  Mar13   0:06  \_ docker-containerd -l /var/run/docker/li
root     12615  0.0  0.1 200496  5424 ?        Sl   14:38   0:00      \_ docker-containerd-shim c5637e5bbdcb
root     12629  3.5  1.6 297460 63340 ?        Ssl  14:38   0:00      |   \_ /home/weave/weaver --port=6783
root     12910  0.0  0.0      0     0 ?        Z    14:38   0:00      |       \_ [launch.sh] <defunct>
[...]
```

Sample output:

```
test #5 "run_on mct-0.us-central1-a.weave-net ps aux | grep -c '[d]efunct'" failed:
	expected "0"
	got "1"
test #6 "run_on mct-1.us-central1-a.weave-net ps aux | grep -c '[d]efunct'" failed:
	expected "0"
	got "1"
test #7 "run_on mct-2.us-central1-a.weave-net ps aux | grep -c '[d]efunct'" failed:
	expected "0"
	got "1"
3 of 8 tests failed in 62.012s.
```

test/840_weave_kube_3_test.sh

@@ -52,6 +52,11 @@ assert_raises "run_on $HOST1 $PING $HOST2EXPIP"
 assert_raises "run_on $HOST2 $PING $HOST1EXPIP"
 assert_raises "run_on $HOST3 $PING $HOST2EXPIP"
 
+# Ensure we do not generate any defunct process (e.g. launch.sh) after starting weaver:
+assert "run_on $HOST1 ps aux | grep -c '[d]efunct'" "0"
+assert "run_on $HOST2 ps aux | grep -c '[d]efunct'" "0"
+assert "run_on $HOST3 ps aux | grep -c '[d]efunct'" "0"
+
 # See if we can get some pods running that connect to the network
 run_on $HOST1 "kubectl run hello --image=weaveworks/hello-world --replicas=3"