Ensure that stderr is caught / $prg=testssl.sh

As suggested in #2780 this PR redirects all stderr on a perl level
to stdout so that we should see all errors which we miss because
we can't redirect (>/dev/null in check) or we haven't redirected yet.

Also old checks are imporved, so that $prg is used as in newer
checks.

.github/workflows/unit_tests_macos.yml

@@ -51,4 +51,4 @@ jobs:
 
       - name: run it
         run: |
-          prove -v t
+          prove -v t 2>&1