Stokhos simplify testing

[romintomasetti]

@trilinos/Stokhos

Motivation

HIP is not tested as thoroughly as other execution spaces. This is a proposal that should ease tester life when he/she wants to add additional execution spaces for a test. This should reduce the amount of copy/paste.

This PR introduces a CMake function that will loop over the list of given spaces, and for each space check that the required variable is defined. If it is defined, the test is added, otherwise it is not.

To do if @etphipp approves the approach:

• Check that the UnitTest/CMakeLists.txt has been simplified everywhere with STOKHOS_ADD_EXECUTABLE_AND_TEST
• Check if there are other opportunities to unify entrypoints into a single entrypoint, like I did for packages/stokhos/test/UnitTest/Stokhos_TpetraCrsMatrixMPVectorUnitTest_*.cpp
• Add missing Stokhos_<test-name>_HIP.cpp if still required

[romintomasetti]

@etphipp Could you please (in)validate this new approach ? This would help us increase the coverage of the testing for HIP 😃

[etphipp]

Looks great, thanks!