Mitigate flaky tests. (#7749)

* Skip non-increasing test with external memory when subsample is used.
* Increase bin numbers for boost from prediction test. This mitigates the effect of
  non-deterministic partitioning.

tests/python-gpu/test_gpu_data_iterator.py

@@ -22,7 +22,7 @@ def test_gpu_single_batch() -> None:
     strategies.integers(0, 13),
     strategies.booleans(),
 )
-@settings(deadline=None)
+@settings(deadline=None, print_blob=True)
 def test_gpu_data_iterator(
     n_samples_per_batch: int, n_features: int, n_batches: int, subsample: bool
 ) -> None: