Add more test cases for mergeability check scenarios

Given this issue (#215109 (closed)) about a bug in mergeability check related to conflict and conflict resolution, it made me realize that the integration/feature specs that we have doesn't cover it (checking of merge status) because nothing failed.

The flow is:

1. User opens MR with changes and conflicts with target branch.
2. User views MR page. Should see that the MR can't be merged due to conflict.
3. User make changes to resolve conflict and pushes to MR branch.
4. User views MR page. Should see that the MR can now be merged.

We can also look into other cases that we can add here to ensure that they don't break.