[TF FE Tests] Change HSVToRGB Unit Test To Test Grayscale Image (#24935)

### Details:
- Currently, the unit test for HSVToRGB is creating a tensor of all ones
in the Grayscale special case as mentioned in
[#24875](https://github.com/openvinotoolkit/openvino/pull/24875). Since
the images are expected to be in HSV and not RGB format, this will
create images that are green, and not gray. Grayscale in HSV can be
achieved by creating a tensor with HSV entries of [0, 0, .5].

### Tickets:
 - N/A

---------

Co-authored-by: Roman Kazantsev <roman.kazantsev@intel.com>

tests/layer_tests/tensorflow_tests/test_tf_HSVToRGB.py

@@ -18,7 +18,7 @@ class TestHSVToRGB(CommonTFLayerTest):
         elif self.special_case == "Grayscale Image":
             images_shape = inputs_info['images:0']
             inputs_data = {}
-            inputs_data['images:0'] = np.ones(images_shape).astype(self.input_type) * np.random.rand()
+            inputs_data['images:0'] = np.broadcast_to([0, 0, 0.5], images_shape).astype(self.input_type)
         else:
             images_shape = inputs_info['images:0']
             inputs_data = {}