Skip mosaic test under pytest that fails with empty ptx capture.
Should resolve CI failure in https://github.com/jax-ml/jax/actions/runs/28201993460/job/83545625909
```
________________________ FragmentedArrayTest.test_math0 ________________________
[gw11] linux -- Python 3.12.13 /usr/bin/python3.12
tests/mosaic/gpu_test.py:4970: in test_math
self.assertNotEqual(ptx, ptx_approx)
E AssertionError: '' == ''
```
PiperOrigin-RevId: 938784635
