nixos/tests/machinectl: Create /usr directory

Since #488508 was merged, systemd nspawn now expects /usr to exist
in the rootfs before it will start a container.

Create the /usr directory in the relevant containers throughout this
test.

nixos/tests/systemd-machinectl.nix

@@ -106,6 +106,8 @@ let
       }
     ];
 
+    extraCommands = "mkdir -p usr";
+
     contents = [
       {
         source = containerSystem + "/etc/os-release";
@@ -171,6 +173,7 @@ in
 
       systemd.tmpfiles.rules = [
         "d /var/lib/machines/shared-decl 0755 root root - -"
+        "d /var/lib/machines/shared-decl/usr 0755 root root - -"
       ];
       systemd.nspawn.shared-decl = {
         execConfig = {
@@ -228,7 +231,7 @@ in
     machine.wait_until_succeeds("systemctl -M shared-decl is-active default.target");
     machine.succeed("machinectl stop shared-decl");
 
-    machine.succeed("mkdir -p ${containerRoot}");
+    machine.succeed("mkdir -p ${containerRoot}/usr");
 
     # start container with shared nix store by using same arguments as for systemd-nspawn@.service
     machine.succeed("systemd-run systemd-nspawn --machine=${containerName} --network-veth -U --bind-ro=/nix/store ${containerSystem}/init")