test: specs for picker.custom

Author: 2KAbhishek

tests/picker_spec.lua

@@ -26,4 +26,11 @@ describe('utils.picker', function()
             assert.are.equals(type(picker.select_file), 'function')
         end)
     end)
+
+    describe('custom', function()
+        it('checks if function exists', function()
+            assert.is_not_nil(picker.custom)
+            assert.are.equals(type(picker.custom), 'function')
+        end)
+    end)
 end)