Make the media sideload functions testable and add tests

[hellofromTonya]

Goals:

1. Make the media "sideload" functions testable
2. Provide code coverage for these functions

To accomplish this:

• These functions will first need to be refactored.
• Then unit tests can be built to provide code coverage.

Sideload functions include:

• media_handle_sideload()
• media_sideload_image()

A guide for the refactoring work:

• All changes will focus on testability
• No changes should impact or alter the current behavior, extensibility, or backwards compatibility
• Care will be given to maintainability and readability (i.e. code quality)

[hellofromTonya]

For reference, the drivers for this ticket were discussed in #core-media slack (links below) initiated by Ticket #50972.

Links to slack discussions prior to opening this ticket:

• ​Comments by @johnbillion

  Might be difficult for the same reason the new filter in media_sideload_image didn't get a test, those functions aren't very testable without refectoring
  

  The sideloading functions could all do with a refactor to make them testable
  

• ​Discussion in Media meeting starts here.