Fix flaky python test

Fix the flaky python test. Moving the file instead of truncating it seems to solve the problem. Maybe the write was not properly synced to disk.

The advantage of moving the file is that if the test runs we can later inspect it.

filebeat/tests/system/test_reload_inputs.py

@@ -105,9 +105,9 @@ def test_start_stop(self):
 
         self.wait_until(lambda: self.output_lines() == 1)
 
-        # Remove input
-        with open(self.working_dir + "/configs/input.yml", 'w') as f:
-            f.write("")
+        # Remove input by moving the file
+        # we keep it around to help debugging
+        os.rename(self.working_dir + "/configs/input.yml", self.working_dir + "/configs/input.yml.disabled")
 
         # Wait until input is stopped
         self.wait_until(