Fix #5335 - Multiple Media Picker not available as macro parameter

[nul800sebastiaan]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #5335

Description

Seemingly due to an oversight, multiple media picker was not available for macro parameters. This corrects that problem.

Test steps:

• Add a Partial View Macro File from snippet "Gallery"
• Go to the macro this has created and enable it to be used in a RTE
• Go to the macro parameters and add a new parameter with the name "Media Picker", the alias mediaIds and of type "Multiple Media Picker"
• Add the "Gallery" macro in a RTE
• You get asked to pick one or more items, make sure you can pick multiple items
• When you're done picking, the macro preview should show you the picked items

[warrenbuckley]

Whilst I review/test this out @nul800sebastiaan can you please fix up the failing unit test :)

[warrenbuckley]

Tested, all works 👍