test-sst-core uses wrong Python on Ubuntu 20.04

[jwilso]

New Issue for sst-core

1 - Detailed description of problem or enhancement Because Ubuntu 20.04 uses only Python3 by default, there is no 'python' executable available, it must be explicitly invoked as 'python3'. sst-test-core looks for 'python', and fails if it is not found.

2 - Describe how to reproduce the issue Run sst-test-core on Ubuntu 20.04 in default configuration.

3 - What Operating system(s) and versions Linux, Ubuntu 20.04

4 - What version of external libraries (Boost, MPI) N/A

5 - Provide sha1 of all relevant sst repositories (sst-core, sst-elements, etc) SST Release 11.0

6 - Fill out Labels, Milestones, and Assignee fields as best possible