Don't leak current VaadinSession in test

Change-Id: Ib4ae44855a1a14ead5217e6f5d1c0d3330747a2f

server/tests/src/com/vaadin/tests/server/component/abstractsinglecomponentcontainer/RemoveFromParentLockingTest.java

                     "Cannot remove from parent when the session is not locked."
                             + " Furthermore, there is another locked session, indicating that the component might be about to be moved from one session to another.",
                     e.getMessage());
+        } finally {
+            VaadinSession.setCurrent(null);
         }
     }
 
             notLockedComponent.addComponent(lockedComponent);
         } catch (AssertionError e) {
             // All is fine, don't care about the exact wording in this case
+        } finally {
+            VaadinSession.setCurrent(null);
         }
     }