test that large packets can be transmitted over weave

[rade]

change in coverage:

$ diff /tmp/coverage-before.txt /tmp/coverage-after.txt 
372c372
< github.com/weaveworks/weave/router/connection_maker.go:161:   queryLoop           87.5%

---
> github.com/weaveworks/weave/router/connection_maker.go:161:   queryLoop           100.0%
411c411
< github.com/weaveworks/weave/router/ethernet_decoder.go:33:    sendICMPFragNeeded      0.0%

---
> github.com/weaveworks/weave/router/ethernet_decoder.go:33:    sendICMPFragNeeded      88.9%
419c419
< github.com/weaveworks/weave/router/forwarder.go:78:       forward             57.9%

---
> github.com/weaveworks/weave/router/forwarder.go:78:       forward             63.2%
549c549
< github.com/weaveworks/weave/router/router.go:147:     handleCapturedPacket        81.5%

---
> github.com/weaveworks/weave/router/router.go:147:     handleCapturedPacket        85.2%
609c609
< total:                                (statements)            82.2%

---
> total:                                (statements)            82.5%

(the connection_maker improvement is just noise)