Created some basic tests of getdominantcolors functionality

Tests/test_image_getdominantcolors.py

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+from .helper import hopper
+
+
+def test_getdominantcolors():
+    def getdominantcolors(mode, numcolors=None):
+        im = hopper(mode)
+
+        if numcolors:
+            colors = im.getdominantcolors(numcolors)
+        else:
+            colors = im.getdominantcolors()
+        return len(colors)
+
+    assert getdominantcolors("F") == 3
+    assert getdominantcolors("I") == 3
+    assert getdominantcolors("L") == 3
+    assert getdominantcolors("P") == 3
+    assert getdominantcolors("RGB") == 3
+    assert getdominantcolors("YCbCr") == 3
+    assert getdominantcolors("CMYK") == 3
+    assert getdominantcolors("RGBA") == 3
+    assert getdominantcolors("HSV") == 3