SOS v1.5.2+ updates (reduce types, C++, wtime)

[davidozog]

This incorporates SOS changes from the following PRs:

• Remove unsupported signed reductions tests for v1.5 Sandia-OpenSHMEM/SOS#1098 - removes unsupported signed reductions tests for v1.5
• tests: Replace shmemx_wtime() with tests_sos_wtime Sandia-OpenSHMEM/SOS#1096 - provides a fallback when shmemx_wtime is not available
• Missing reduction types for C++ templates Sandia-OpenSHMEM/SOS#1095 - adds missing reduction types for C++ templates

After merging, we can close #26

[davidozog]

@kholland-intel - these changes will be helpful/necessary for Sandia-OpenSHMEM/SOS#1106

[kholland-intel]

LGTM, thanks for bringing this PR back to my attention 👍

[davidozog]

Oops - I think we should bring the shmemx_team_node test here now, otherwise it could be lost in the submodule swap on PR Sandia-OpenSHMEM/SOS#1106.

[davidozog]

@kholland-intel - could you please do a sanity check review of 3154039?

[kholland-intel]

@kholland-intel - could you please do a sanity check review of 3154039?

diff looks good. 👍