Create a new UeventSender.sender object for every uevent call

The UeventSender.sender class uses libudev under the covers. libudev's
documentation (referenced below) states:

> Only a single specific thread may operate on a given object during its
> entire lifetime. It's safe to allocate multiple independent objects
> and use each from a specific thread in parallel. However, it's not
> safe to allocate such an object in one thread, and operate or free it
> from any other, even if locking is used to ensure these threads don't
> operate on it at the very same time.

Therefore, if a test is multi-threaded and one thread calls to add
devices the object might be created on that thread, then later another
thread might remove the device and attempt to use the same object.

For example, this can very easily happen in a ROS python-based
environment where ROS spawns a new thread for each callback.

Reference: https://www.freedesktop.org/software/systemd/man/latest/libudev.html

src/umockdev.vala

@@ -825,11 +825,10 @@ public class Testbed: GLib.Object {
      */
     public void uevent (string devpath, string action)
     {
-        if (this.ev_sender == null) {
-            debug("umockdev_testbed_uevent: lazily initializing uevent_sender");
-            this.ev_sender = new UeventSender.sender(this.root_dir);
-            assert(this.ev_sender != null);
-        }
+        UeventSender.sender ev_sender = null;
+        ev_sender = new UeventSender.sender(this.root_dir);
+        assert(ev_sender != null);
+
         debug("umockdev_testbed_uevent: sending uevent %s for device %s", action, devpath);
 
         var uevent_path = Path.build_filename(this.root_dir, devpath, "uevent");
@@ -839,7 +838,7 @@ public class Testbed: GLib.Object {
         } catch (FileError e) {
             debug("uevent: devpath %s has no uevent file: %s",  devpath, e.message);
         }
-        this.ev_sender.send(devpath, action, properties);
+        ev_sender.send(devpath, action, properties);
     }
 
     /**
@@ -1660,7 +1659,6 @@ public class Testbed: GLib.Object {
     private Regex re_record_val;
     private Regex re_record_keyval;
     private Regex re_record_optval;
-    private UeventSender.sender? ev_sender = null;
     private HashTable<string,int> dev_fd;
     private HashTable<string,ScriptRunner> dev_script_runner;
     private SocketServer socket_server = null;