Cause TestLib.pm to define $windows_os in all branches.

Back-port of a part of commit 690ed2b76a that
I'd depended on without realizing that it was only added recently.  Since
it seems entirely likely that other such tests will need to be back-patched
in future, providing the flag seems like a better answer than just putting
a test in-line.

Per buildfarm.

src/test/perl/TestLib.pm

@@ -26,6 +26,7 @@ our @EXPORT = qw(
 
   $tmp_check
   $log_path
+  $windows_os
 );
 
 use Cwd;
@@ -38,6 +39,8 @@ use SimpleTee;
 
 use Test::More;
 
+our $windows_os = $Config{osname} eq 'MSWin32' || $Config{osname} eq 'msys';
+
 # Open log file. For each test, the log file name uses the name of the
 # file launching this module, without the .pl suffix.
 our ($tmp_check, $log_path);