user session agent panic'ing in tests

[Ian Johnson]

See:

Nov 30 20:06:23 nov301937-780871 systemd[860]: Reloading.
Nov 30 20:06:23 nov301937-780871 snap[23633]: Exiting on terminated.
Nov 30 20:06:23 nov301937-780871 systemd[860]: Stopping snapd user session agent...
Nov 30 20:06:23 nov301937-780871 snap[23633]: panic: runtime error: invalid memory address or nil pointer dereference
Nov 30 20:06:23 nov301937-780871 snap[23633]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x78 pc=0x563690201448]
Nov 30 20:06:23 nov301937-780871 snap[23633]: goroutine 9 [running]:
Nov 30 20:06:23 nov301937-780871 snap[23633]: github.com/godbus/dbus.(*Conn).Close(0x0, 0x0, 0x0)
Nov 30 20:06:23 nov301937-780871 snap[23633]: /home/gopath/src/github.com/snapcore/snapd/vendor/github.com/godbus/dbus/conn.go:240 +0x28
Nov 30 20:06:23 nov301937-780871 snap[23633]: github.com/snapcore/snapd/usersession/agent.(*SessionAgent).shutdownServerOnKill(0xc000364c80, 0x0, 0x0)
Nov 30 20:06:23 nov301937-780871 snap[23633]: /home/gopath/src/github.com/snapcore/snapd/usersession/agent/session_agent.go:279 +0xab
Nov 30 20:06:23 nov301937-780871 snap[23633]: gopkg.in/tomb%2ev2.(*Tomb).run(0xc000364cb0, 0xc00034b3f0)
Nov 30 20:06:23 nov301937-780871 snap[23633]: /home/gopath/src/github.com/snapcore/snapd/vendor/gopkg.in/tomb.v2/tomb.go:163 +0x2d
Nov 30 20:06:23 nov301937-780871 snap[23633]: created by gopkg.in/tomb%2ev2.(*Tomb).Go
Nov 30 20:06:23 nov301937-780871 snap[23633]: /home/gopath/src/github.com/snapcore/snapd/vendor/gopkg.in/tomb.v2/tomb.go:159 +0xc9
Nov 30 20:06:23 nov301937-780871 systemd[860]: snapd.session-agent.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Nov 30 20:06:23 nov301937-780871 systemd[860]: snapd.session-agent.service: Failed with result 'exit-code'.

Full spread log from an unrelated spread test: https://pastebin.ubuntu.com/p/7Y5MYZ58zS/

Note that the panic could be related to any of the previous tests that were executed from that run, which can be found looking at the very end of the log at the "runs" file output.