[SPARK-20596][ML][TEST] Consolidate and improve ALS recommendAll test cases

Existing test cases for `recommendForAllX` methods (added in [SPARK-19535](https://issues.apache.org/jira/browse/SPARK-19535)) test `k < num items` and `k = num items`. Technically we should also test that `k > num items` returns the same results as `k = num items`.

## How was this patch tested?

Updated existing unit tests.

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #17860 from MLnick/SPARK-20596-als-rec-tests.