Sort test: check stability (device policies)

[dmitriy-sobolev]

Summary:
stable_sort.pass does not check stability when the algorithm is executed with device policies.

Stability, however, is checked in stable_sort_by_key.pass, which shares the same implementation, but it is an implementation details we should not rely on.

There is a TODO in the test:

oneDPL/test/parallel_api/algorithm/alg.sorting/alg.stable.sort/stable_sort.pass.cpp

Line 36 in f49feb4

// TODO: add a test for stability