Update test_grayscott.py

Author: pancetta

pySDC/projects/SDC_showdown/tests/test_grayscott.py

@@ -1,8 +1,8 @@
 import pytest
 
-
+@pytest.mark.slow
 @pytest.mark.petsc
-@pytest.mark.timeout(600)
+@pytest.mark.timeout(900)
 def test_grayscott():
     from pySDC.projects.SDC_showdown.SDC_timing_GrayScott import main