Ensure we test providing a custom policy_hub for each public cub dispatcher

[bernhardmgruber]

For the internal restructuring to support the new tuning API, we are touching a lot of the dispatching logic. We should ensure every publicly accessible CUB dispatcher has at least a unit test where a custom policy_hub is provided.

These tests are easy to generate using an AI with a prompt like:

Look at commit 186c97c adding a custom policy hub test for DispatchRadixSort.
Implement a similar test for XXX, run the tests, and fix any errors.

• Test passing a custom policy to DispatchAdjacentDifference, DispatchMergeSort, DispatchScan, DispatchBatchMemcpy #7289
• Test passing a custom policy to DispatchHistogram #7288
• Test passing a custom policy to DispatchRadixSort #7170
• Test passing a custom policy to DispatchReduce #6577
• Test passing a custom policy to DispatchReduceByKey #7310
• Test passing a custom policy to DeviceRleDispatch #7314
• Test passing a custom policy to DispatchSegmentedRadixSort #7308
• Test passing a custom policy to DispatchSegmentedReduce #7311
• Test passing a custom policy to DispatchSegmentedSort #7307
• Test passing a custom policy to DispatchSelectIf #7294
• Test passing a custom policy to DispatchScanByKey #7309
• Test passing a custom policy to DispatchThreeWayPartitionIf #7295
• Test passing a custom policy to DispatchUniqueByKey #7296