Fix issue #2649, create unit tests for darkness-checking logic using six images (#5096)

* add images and test too dark

* add dark and proper images and check if too dar

* test: add refer

Co-authored-by: Ruofan <u7307578@anu.edu.au>

app/src/test/kotlin/fr/free/nrw/commons/utils/ImageUtilsTest.kt

@@ -51,7 +51,30 @@ class ImageUtilsTest {
 
     @Test
     fun testCheckIfImageIsTooDarkCaseException() {
-        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark(""), ImageUtils.IMAGE_OK)
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("")
+            , ImageUtils.IMAGE_OK)
+    }
+
+    // Refer: testCheckIfImageIsTooDarkCaseException()
+    @Test
+    fun testCheckIfProperImageIsTooDark() {
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/ok1.jpg")
+            , ImageUtils.IMAGE_OK)
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/ok2.jpg")
+            , ImageUtils.IMAGE_OK)
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/ok3.jpg")
+            , ImageUtils.IMAGE_OK)
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/ok4.jpg")
+            , ImageUtils.IMAGE_OK)
+    }
+
+    // Refer: testCheckIfImageIsTooDarkCaseException()
+    @Test
+    fun testCheckIfDarkImageIsTooDark() {
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/dark1.jpg")
+            , ImageUtils.IMAGE_DARK)
+        Assert.assertEquals(ImageUtils.checkIfImageIsTooDark("src/test/resources/ImageTest/dark2.jpg")
+            , ImageUtils.IMAGE_DARK)
     }
 
     @Test