runtests: consider warnings fatal and error on them

To help us detect and fix warnings in this script easier and faster.

Assisted-by: Jakob Hirsch

Ref: #10206
Closes #10208

tests/runtests.pl

@@ -75,7 +75,8 @@ BEGIN {
 }
 
 use strict;
-use warnings;
+# Promote all warnings to fatal
+use warnings FATAL => 'all';
 use Cwd;
 use Digest::MD5 qw(md5);
 use MIME::Base64;