fix: initialize socketPath to prevent stale cache (#491)

`daemonSuite.socketPath` appears to be assigned outside of the test, but
only when it has a default value, which results in a stale `socketPath`
value in this test.

Running `go test -count=2 -v ./internals/daemon -check.v` with this fix
now passes.

Fixes #282.

internals/daemon/daemon_test.go

@@ -69,6 +69,7 @@ func (s *daemonSuite) SetUpTest(c *C) {
 		c.Fatalf("cannot start reaper: %v", err)
 	}
 
+	s.socketPath = ""
 	s.pebbleDir = c.MkDir()
 	s.statePath = filepath.Join(s.pebbleDir, cmd.StateFile)
 	systemdSdNotify = func(notif string) error {