whisper: TestSimulation fails when existing node running

[nolash]

System information

Geth version: 1.8.0-unstable
OS & Version: Linux 4.13.7-1-ARCH
Commit hash : (if c6069a627c42c21fc02d0770d39db9a9be45b180)

Expected behaviour

Test should pass

Actual behaviour

If node is already running, p2p.Server.Start() fails because port is taken.

Steps to reproduce the behaviour

go test -v -cpu 4 ./whisper/whisperv5/ -run Simulation

Backtrace

=== RUN   TestSimulation
--- FAIL: TestSimulation (0.00s)
	peer_test.go:159: failed to start server 0.

[original-brownbear]

Gave this a shot in #15687 :)