add some context to help debug Appveyor error

Author: mr-c

tests/test_examples.py

@@ -937,6 +937,10 @@ def test_env_filtering():
     error_code, stdout, stderr = get_main_output(
         [get_data(test_file)])
 
-    assert "completed success" in stderr
-    assert error_code == 0
-    assert json.loads(stdout)['env_count'] == 4
+    assert "completed success" in stderr, (error_code, stdout, stderr)
+    assert error_code == 0, (error_code, stdout, stderr)
+    if onWindows:
+        target = 5
+    else:
+        target = 4
+    assert json.loads(stdout)['env_count'] == target, (error_code, stdout, stderr)