Make some perl scripts usable with git bisect run

For that they need to return between 0 and 124 on error, while die returns
255, causing bisect-run to abort.

tests/scripts/key-exchanges.pl

@@ -33,7 +33,8 @@ my @kexes = split( /\s+/, `sed -n -e '$sed_cmd' $config_h` );
 system( "cp $config_h $config_h.bak" ) and die;
 sub abort {
     system( "mv $config_h.bak $config_h" ) and warn "$config_h not restored\n";
-    die $_[0];
+    warn $_[0];
+    exit 1;
 }
 
 for my $kex (@kexes) {