Don't use native methods in TestLib::slurp_file on Msys

Commit 114541d58e has upset some buildfarm members running Msys, that
weren't previously having problems, so the use of native Windows methods
to open files is restricted by this patch to only MSVC builds.

src/test/perl/TestLib.pm

@@ -400,7 +400,7 @@ sub slurp_file
 	my ($filename) = @_;
 	local $/;
 	my $contents;
-	if (!$windows_os)
+	if ($Config{osname} ne 'MSWin32')
 	{
 		open(my $in, '<', $filename)
 		  or die "could not read \"$filename\": $!";