Fixed return status for posit_command_run and zsh tests

Tests were failing on zsh due to unquoted glob patterns on test
files only, now they're fixed.

posit_command_run had a critical bug that prevented the proper
exit status to be reported at the end of a test (reports for
each individual test were still OK). For the fix, the posit_all_*
group of functions now gained the responsibility of calling
posit_process as well.