create tests to get shares by id & do accept and decline them

[individual-it]

this tests are needed for ocis but need to be added in core repo

• get share by valid id ([Tests-Only] add api tests for accessing shares by id core#38757)
• accept share ([Tests-Only] add api tests for accessing shares by id core#38757)
• decline share ([Tests-Only] add api tests for accessing shares by id core#38757)
• try all actions with valid and invalid ids
• check response of of all see Getting a share by id returns state="0" cs3org/reva#1684

[phil-davis]

Note: tests should also run and pass in oC10

[dpakach]

@individual-it I added some tests in owncloud/core#38757, I cannot think of any other scenarios regarding this. can we close this?

[individual-it]

CC @kulmann I'm closing this for now, please reopen or create a new issue if you feel more tests are needed