Skip Screen Curtain unit test if active (#17467)

Follow up to #17413 (comment)

Summary of the issue:
The screen curtain unit tests fail if the screen curtain (or another colour transform e.g. contrast mode) is active.
Instead these test should be skipped.
Alternatively, they could be run in debug mode to do the test without collecting the failure

tests/unit/test_visionEnhancementProviders/test_magnificationAPI.py

@@ -7,7 +7,7 @@
 
 import unittest
 
-from visionEnhancementProviders.screenCurtain import Magnification, TRANSFORM_BLACK
+from visionEnhancementProviders.screenCurtain import Magnification, TRANSFORM_BLACK, MAGCOLOREFFECT
 
 
 class _Test_MagnificationAPI(unittest.TestCase):
@@ -19,6 +19,29 @@ def tearDown(self):
 
 
 class Test_ScreenCurtain(_Test_MagnificationAPI):
+	def _isIdentityMatrix(self, magTransformMatrix: MAGCOLOREFFECT) -> bool:
+		for i in range(5):
+			for j in range(5):
+				if i == j:
+					if magTransformMatrix.transform[i][j] != 1:
+						return False
+				else:
+					if magTransformMatrix.transform[i][j] != 0:
+						return False
+		return True
+
+	def setUp(self):
+		super().setUp()
+		resultEffect = Magnification.MagGetFullscreenColorEffect()
+		if not self._isIdentityMatrix(resultEffect):
+			# If the resultEffect is not the identity matrix, skip the test.
+			# This is because a full screen colour effect is already set external to testing.
+			self.skipTest(
+				f"{resultEffect=}, should be identity matrix. "
+				"Full screen colour effect set external to tests. ",
+			)
+		return
+
 	def test_setAndConfirmBlackFullscreenColorEffect(self):
 		result = Magnification.MagSetFullscreenColorEffect(TRANSFORM_BLACK)
 		self.assertTrue(result)
@@ -32,18 +55,6 @@ def test_setAndConfirmBlackFullscreenColorEffect(self):
 						msg=f"i={i}, j={j}, resultEffect={resultEffect}",
 					)
 
-	def test_getDefaultIdentityFullscreenColorEffect(self):
-		resultEffect = Magnification.MagGetFullscreenColorEffect()
-		for i in range(5):
-			for j in range(5):
-				with self.subTest(i=i, j=j):
-					# The transform matrix should be the identity matrix
-					self.assertEqual(
-						int(i == j),
-						resultEffect.transform[i][j],
-						msg=f"i={i}, j={j}, resultEffect={resultEffect}",
-					)
-
 
 class Test_Mouse(_Test_MagnificationAPI):
 	def test_MagShowSystemCursor(self):