Manifests-api: Added few tests related to manifests

Added the below tests:
* test_positive_create_1
* test_positive_delete_1
* test_negative_create_1
> I assumed it to be a **typo** and fixed it for `delete_manifest function` too.

Our `requests` wrapper functions exhibit buggy behaviour when uploading data. The existing calls to `_call_requests_*` are in place to work around this buggy behaviour. Ideally, `client.post` (and similar) should be fixed.

Please drop this change. I don't believe the call to `client.post` is a typo.