fix: use imagemagick's "convert" to generate a solid image for testing

Due to using file_arg instead of file_in, the previous solution was unnecessarily generating
plasma image and displayed a wrong input image in visual report, now it is fixed

It also makes the code simpler and more consistent with generation of other images

spec/colour_helper.rb

@@ -16,25 +16,12 @@ def generate_test_image_greyscale(at_file_path, width: 600, height: 300)
   def generate_test_image(at_file_path, fill:, width: 600, height: 300)
     fill = Array(fill).flatten
 
-    system(
-      'convert',
-      '-size',
-      "#{width}x#{height}",
-      '-seed',
-      '5432',
-      *fill,
-      at_file_path
-    )
+    generate_image_options = ['convert', '-size', "#{width}x#{height}", '-seed', '5432', *fill, at_file_path]
+    system(*generate_image_options) || raise("Failed to generate image.\nCommand: #{generate_image_options.join(' ')}")
   end
 
-  def solid_colour_image(width, height, colour = 0x000000ff)
-    pb = GdkPixbuf::Pixbuf.new(colorspace: GdkPixbuf::Colorspace::RGB,
-                               has_alpha: false,
-                               bits_per_sample: 8,
-                               width: width,
-                               height: height)
-    pb.fill!(colour)
-    pb
+  def generate_test_image_solid(at_file_path, width: 600, height: 300, colour: '#000000')
+    generate_test_image(at_file_path, width: width, height: height, fill: "canvas:#{colour}")
   end
 
   def crude_average_colour(pixbuf)

spec/morandi_spec.rb

@@ -578,7 +578,10 @@
       end
 
       context 'with a gray image and invalid spots' do
-        let(:file_arg) { solid_colour_image(800, 800, 0x666666ff) }
+        let(:file_in) { 'sample/sample.jpg' }
+        let(:original_image_width) { 800 }
+        let(:original_image_height) { 800 }
+        let(:generate_image) { generate_test_image_solid(file_in, width: 800, height: 800, colour: '#666666') }
         let(:options) { { 'redeye' => [[540, 650], [-100, 100]] } }
 
         it 'should not break or corrupt the image' do
@@ -587,9 +590,9 @@
           expect(File).to exist(file_out)
           expect(processed_image_type).to eq('jpeg')
 
-          expect(crude_average_colour(file_arg.subpixbuf(505, 605, 100, 100))).to be_greyish
-          expect(crude_average_colour(GdkPixbuf::Pixbuf.new(file: file_out).subpixbuf(505, 605, 100,
-                                                                                      100))).to be_greyish
+          expect(crude_average_colour(GdkPixbuf::Pixbuf.new(file: file_in).subpixbuf(505, 605, 100, 100))).to be_greyish
+          expect(crude_average_colour(GdkPixbuf::Pixbuf.new(file: file_out).subpixbuf(505, 605, 100, 100)))
+            .to be_greyish
         end
       end
     end