ColorPicker: improve unit tests

[ciampo]

As originally flagged in #49214 (comment)_, the unit test for ColorPicker could definitely be improved:

• they don't use user-event
• they don't check for the exact color values that are passed to the onChange function (they currently only check that it's a valid hex value)
• they don't check the value of the input fields
• they only interact with the component by simulating a mouse drag over the "canvas", and therefore they don't test all of the different input fields available:
  • The hex input
  • The HSL input/sliders
  • The RGB input/sliders
  • The hue slider
  • The Alpha slider