use tests.sqlite so it doesn't conflict with running db

run_tests.py

@@ -58,8 +58,8 @@ class NovaTestRunner(core.TextTestRunner):
 
 
 if __name__ == '__main__':
-    if os.path.exists("nova.sqlite"):
-        os.unlink("nova.sqlite")
+    if os.path.exists("tests.sqlite"):
+        os.unlink("tests.sqlite")
     c = config.Config(stream=sys.stdout,
                       env=os.environ,
                       verbosity=3,