Fix test_inference_instance_segmentation_head
# What does this PR do?

Current `test_inference_instance_segmentation_head` (in `MaskFormerModelIntegrationTest`) failed in CI.
The expected slice 
```python
[[-1.3738, -1.7725, -1.9365], [-1.5978, -1.9869, -2.1524], [-1.5796, -1.9271, -2.0940]]
```
has precision `4` and `atol` argument (`TOLERANCE`) is also `1e-4`, which makes the difference at the boundary. This is **likely** the cause of test failures. Give more precision for the expected values should fix the issue.

```bash
(Pdb) diff1 # (with original expected values)
0.0001039505
(Pdb) diff2 # (with more precision)
1.4066696e-05
```

(However, I am not able to get the test failure with the original setting, launched manually in a GCP VM.)
_The documentation is not available anymore as the PR was closed or merged._