tests: stop promoting perl warnings to fatal errors

While this is useful on CI to highlight issues in the scripts, it's a
recipe for pain when enabled on users' builds.

Suggested-by: Leon Timmermans
Ref: #16128

tests/runtests.pl

@@ -57,8 +57,7 @@
 # given, this won't be a problem.
 
 use strict;
-# Promote all warnings to fatal
-use warnings FATAL => 'all';
+use warnings;
 use 5.006;
 use POSIX qw(strftime);