Fix use of OSCommand in test suites

[gvoskuilen]

The "public" API for test suites is included in sst_unittest.py and sst_unittest_support.py. sst-unittest's os_simple_command() function provides some of the underlying OSCommand functionality but not enough and so test suites are using OSCommand directly. Either OSCommand() needs to be part of the sst-unittest API or unittest needs a more functional wrapper for it.

E.g.,
os_command(os_cmd, output_file_path=None, error_file_path=None, set_cwd=None, timeout_sec=60)
The function should return OSCommandResult (test suites are using rtn.result, rtn.output, rtn.error).

Also include ability to pass env vars once that is added to OSCommand.
Deprecate os_simple_command() in favor of this.