[12.x] Add test for collapse in collections

[amirmohammadnajmi]

This PR, improvement tests for collapse method in collections:

• Case including numeric and string elements

$data = new $collection([[1], [2], [3], ['foo', 'bar'], new $collection(['baz', 'boom'])]);
$this->assertEquals([1, 2, 3, 'foo', 'bar', 'baz', 'boom'], $data->collapse()->all());

• Case with empty two-dimensional arrays

$data = new $collection([[], [], []]);
$this->assertEquals([], $data->collapse()->all());

• Case with both empty arrays and arrays with elements

$data = new $collection([[], [1, 2], [], ['foo', 'bar']]);
$this->assertEquals([1, 2, 'foo', 'bar'], $data->collapse()->all());

• Case including collections and arrays

$collection = new $collection(['baz', 'boom']);
$data = new $collection([[1], [2], [3], ['foo', 'bar'], $collection]);
$this->assertEquals([1, 2, 3, 'foo', 'bar', 'baz', 'boom'], $data->collapse()->all());