Only run imagehash tests on Linux machines (#4039)

Author: schlunma

setup.cfg

@@ -15,6 +15,7 @@ env =
 log_level = WARNING
 markers =
     installation: test requires installation of dependencies
+    diagnostic_image_output: test produces images that will be compared with imagehash
 
 [coverage:run]
 parallel = true

tests/conftest.py

@@ -1,6 +1,21 @@
 """Configure pytest."""
 
+import sys
+
+import pytest
+
 
 def pytest_addoption(parser):
     """Add option to save imagehashes (used for diagnostic tests)."""
     parser.addoption("--save_imagehashes")
+
+
+def pytest_runtest_setup(item):
+    """Only run diagnostic tests that produce images on Linux machines."""
+    if item.get_closest_marker("diagnostic_image_output") is not None:
+        platform = sys.platform
+        if platform != "linux":
+            pytest.skip(
+                f"Diagnostic tests that produce images are not supported on "
+                f"platform {platform}"
+            )