UI/backend: add more s3gw-ui backend unit tests for weird edge cases

[jecluis]

What the title says.

Although this is far too vague. Requires said edge cases to be defined as acceptance criteria for this issue.

[jecluis]

I believe the s3gw-ui backend has a decent amount of unit tests, albeit I'll concede they may not be testing weird stuff.

That said, pushing it until after v0.23.0.

[Ameenasuhani]

@jecluis I would appreciate if you could please add more clear and concise description as to what edge test case coverage is required here.

[votdev]

Try to imagine as much crazy stuff as possible. Users are sometimes really inventive and do things you can't imagine.