Add Gradient tool test for double clicking to insert a gradient stop

[Sidharth-Singh10]

Closes part of #2488

[Sidharth-Singh10]

@0HyperCube, could you please review the flow of code once?
This is my first time here writing tests.

I included a check for whether if the stop was inserted at the correct position,

let positions: Vec<f64> = updated_gradient.stops.iter().map(|(pos, _)| *pos).collect();
		assert!(
			positions.iter().any(|pos| (pos - 0.5).abs() < 0.1),
			"Expected to find a stop near position 0.5, but found: {:?}",
			positions
		);

but it may not be necessary as the test is just for check for insertion of gradient stop

[0HyperCube]

Looks good; thanks for this contribution.