Improve tests for particle effects

[iche033]

Desired behavior

Better tests for particle effects system, like testing changing params like particle type, size, life time, velocity, duration, etc and verify image output

Alternatives considered

Testing those params in unit tests, which have already been done, but they do not verify the actual image output

Implementation suggestion

Add integration tests and verify generated image, e.g. set color and verify pixel rgb value, or change particle size and verify fewer pixels in the image are particles

Additional context

see comment: https://github.com/ignitionrobotics/ign-rendering/pull/241#issuecomment-778497838