tests: Ensure objects are free'ed at the end of tests

The objects may not be garbage collected otherwise.

tests/capture.py

@@ -17,6 +17,7 @@ c.enumerate()
 devices = c.get_devices()
 
 d = devices[0]
+del devices
 
 d.open_sync()
 
@@ -24,6 +25,9 @@ img = d.capture_sync(True)
 
 d.close_sync()
 
+del d
+del c
+
 width = img.get_width()
 height = img.get_height()