Improve tests for particle effects

## 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