test: Mark test_gpt tests as slow

Mark all the tests in this file as slow, since they take a while.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_gpt.py b/test/py/tests/test_gpt.py
index f707d9f..cb44e1d 100644
--- a/test/py/tests/test_gpt.py
+++ b/test/py/tests/test_gpt.py
@@ -13,6 +13,9 @@
 the test.
 """
 
+# Mark all tests here as slow
+pytestmark = pytest.mark.slow
+
 class GptTestDiskImage(object):
     """Disk Image used by the GPT tests."""