Fix the test

Author: seisman

pygmt/tests/test_helpers.py

@@ -54,7 +54,7 @@ def my_module(**kwargs):
 def test_gmttempfile():
     "Check that file is really created and deleted."
     with GMTTempFile() as tmpfile:
-        assert os.path.exists(tmpfile.name)
+        assert not os.path.exists(tmpfile.name)
     # File should be deleted when leaving the with block
     assert not os.path.exists(tmpfile.name)