tests: root-unmount: Wait for the first QEMU process to finish.

There was a tiny possibility that the first QEMU process would still be
running by the time we launch the second one.

* gnu/build/marionette.scm (marionette-pid): Export.
* gnu/tests/base.scm (run-root-unmount-test)[test]: Add 'waitpid' call.

gnu/build/marionette.scm

@@ -28,6 +28,7 @@
   #:use-module (ice-9 popen)
   #:use-module (ice-9 regex)
   #:export (marionette?
+            marionette-pid
             make-marionette
             marionette-eval
             wait-for-file

gnu/tests/base.scm

@@ -695,7 +695,13 @@ in a loop.  See <http://bugs.gnu.org/26931>.")
 
             ;; Halt the system.
             (marionette-eval '(system* "/run/current-system/profile/sbin/halt")
-                             marionette))
+                             marionette)
+
+            (display "waiting for marionette to complete...")
+            (force-output)
+            (false-if-exception (waitpid (marionette-pid marionette)))
+            (display " done\n")
+            (force-output))
 
           ;; Remove the sockets used by the marionette above to avoid
           ;; EADDRINUSE.