Add more test coverage to apiCustomGroups

[phil-davis]

Add some test scenarios to cover:

• combinations of sharing when there is an ordinary group name and custom group names that are the same (similar to issue Groups cannot be differentiated when sharing #401 but use the API) done in [tests-only] Add more test coverage to apiCustomGroups #427
• combinations of sharing when a folder is shared with an ordinary user/group and sub-folder shared with a custom group. done in [tests-only] Add more test coverage to apiCustomGroups #427
• combinations of sharing when a folder is shared with an custom group and sub-folder shared with an ordinary user/group group. done in [tests-only] Add more test coverage to apiCustomGroups #427
• resharing of a received custom group share by the share receiver - done in [tests-only] Add more test coverage to apiCustomGroups #427
• trying to leave a custom group if you have already left it (see issue Leaving a group you already left shouldn't be possible #414 ) done in [tests-only] Add more test coverage to apiCustomGroups #427

[phil-davis]

PR #427 has been merged.
@SwikritiT I think this issue is finished?

[SwikritiT]

@phil-davis Yes, it is. I think we can close it.