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>

test/py/tests/test_gpt.py

@@ -13,6 +13,9 @@ These tests rely on a 4 MB disk image, which is automatically created by
 the test.
 """
 
+# Mark all tests here as slow
+pytestmark = pytest.mark.slow
+
 class GptTestDiskImage(object):
     """Disk Image used by the GPT tests."""