Fix a condition in test_generate_with_head_masking
Fix a glitch in a condition in `test_generate_with_headmasking`, i.e.

```diff
- if set(head_masking.keys()) < set([*signature.parameters.keys()]):
+ if set(head_masking.keys()) > set([*signature.parameters.keys()]):
    continue
```

This PR also fixes usage of head_mask for bigbird_pegasus and speech2texy models.

**Reviewer:** @patrickvonplaten 