Fix leaking FUSE mounts from tests.

Change-Id: Ia8a0e97e51c445605ecf0fdb219a104e0330ee6a

populate/e2e_test.go

diff --git a/populate/e2e_test.go b/populate/e2e_test.go
index 6cd51d2..3b51536 100644
--- a/populate/e2e_test.go
+++ b/populate/e2e_test.go
@@ -124,12 +124,12 @@
 
 	root := fs.NewMultiFS(service, fix.cache, opts)
 	fuseOpts := nodefs.NewOptions()
-	server, _, err := nodefs.MountRoot(filepath.Join(dir, "mnt"), root, fuseOpts)
+	fix.fsServer, _, err = nodefs.MountRoot(filepath.Join(dir, "mnt"), root, fuseOpts)
 	if err != nil {
 		return nil, err
 	}
 
-	go server.Serve()
+	go fix.fsServer.Serve()
 
 	return &fix, nil
 }