Do not use shell built-ins

As we no longer use sequences of commands in the Makefile, AppVeyor's make does
not actually invoke a shell to call test.pl. Hence invoking "echo" from within
test.pl fails.

From the `make` documentation: "When it is time to execute commands to update a
target, they are executed by making a new subshell for each line. (In practice,
make may take shortcuts that do not affect the results.)" It seems these
"shortcuts" are being taken by AppVeyor's make.

Author: tautschnig

regression/test.pl

@@ -42,8 +42,10 @@ ($$$$$)
     }
   }
 
-  system "echo EXIT=$exit_value >>'$name/$output'";
-  system "echo SIGNAL=$signal_num >>'$name/$output'";
+  open my $FH, ">>$name/$output";
+  print $FH "EXIT=$exit_value\n";
+  print $FH "SIGNAL=$signal_num\n";
+  close $FH;
 
   if($signal_num == 2) {
     print "\nProgram under test interrupted; stopping\n";