test-runner: only remove /tmp files if they exist

This bit of code was throwing exceptions if a test cleaned up files that
test-runner was expecting to clean up. Specifically testHotspot swaps out
main.conf and PSK files many times. This led to the exception being thrown,
caught, and ignored but further on test-runner would print:

"File _X_ not cleaned up!"

Now the files will be checked if they exist before trying to remove it.

tools/run-tests

@@ -863,7 +863,7 @@ def post_test(ctx, to_copy):
 		for f in to_copy:
 			if os.path.isdir('/tmp/' + f):
 				shutil.rmtree('/tmp/' + f)
-			else:
+			elif os.path.exists('/tmp/' + f):
 				os.remove('/tmp/' + f)
 
 		Process(['ip', 'link', 'set', 'lo', 'down']).wait()