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/nag.t

@@ -28,7 +28,7 @@
 
 use strict;
 use warnings;
-use Test::More tests => 9;
+use Test::More tests => 10;
 
 # Create the rc file.
 if (open my $fh, '>', 'nag.rc')
@@ -58,6 +58,9 @@ unlike (qx{../task rc:nag.rc do 1}, qr/NAG/, 'do due:yesterday -> no nag');
 unlink 'pending.data';
 ok (!-r 'pending.data', 'Removed pending.data');
 
+unlink 'undo.data';
+ok (!-r 'undo.data', 'Removed undo.data');
+
 unlink 'nag.rc';
 ok (!-r 'nag.rc', 'Removed nag.rc');