Unit Tests

- All unit tests most now clean up a residual undo.data file, so the
  number of tests increased, but with no benefit.

src/tests/bug.sort.t

@@ -28,7 +28,7 @@
 
 use strict;
 use warnings;
-use Test::More tests => 5;
+use Test::More tests => 6;
 
 # Create the rc file.
 if (open my $fh, '>', 'bug_sort.rc')
@@ -54,6 +54,9 @@ like ($output, qr/three.*one.*two/msi, 'list did not hang after pri:H on 1');
 unlink 'pending.data';
 ok (!-r 'pending.data', 'Removed pending.data');
 
+unlink 'undo.data';
+ok (!-r 'undo.data', 'Removed undo.data');
+
 unlink 'bug_sort.rc';
 ok (!-r 'bug_sort.rc', 'Removed bug_sort.rc');