tests: encourage to release the lock after completing the setup

Many functions and especially standalone tools takes the lock itself, so
to prevent deadlocks, as a rule execute the tests with lock released.

Also reload qubes.xml before cleanup.

tests/__init__.py

@@ -187,8 +187,8 @@ class SystemTestsMixin(object):
 
         .. warning::
             This method instantiates QubesVmCollection acquires write lock for
-            it. You can use is as :py:attr:`qc`. Do not release the lock
-            yourself.
+            it. You can use is as :py:attr:`qc`. You can (and probably
+            should) release the lock at the end of setUp in subclass
         '''
 
         super(SystemTestsMixin, self).setUp()
@@ -205,6 +205,10 @@ class SystemTestsMixin(object):
     def tearDown(self):
         super(SystemTestsMixin, self).tearDown()
 
+        try: self.qc.lock_db_for_writing()
+        except qubes.qubes.QubesException: pass
+        self.qc.load()
+
         self.remove_test_vms()
 
         self.qc.save()

tests/vm_qrexec_gui.py

@@ -47,6 +47,7 @@ class TC_00_AppVM(qubes.tests.SystemTestsMixin, qubes.tests.QubesTestCase):
             template=self.qc.get_default_template())
         self.testvm2.create_on_disk(verbose=False)
         self.qc.save()
+        self.qc.unlock_db()
 
 
     def test_000_start_shutdown(self):