Extend test coverage for Dumpvalue.pm

Extend test coverage for Dumpvalue.pm

Improve coverage since version 1.18.

$ cat Dumpvalue.original.coverage
--------------------- ------ ------ ------ ------ ------ ------ ------
File                    stmt   bran   cond    sub    pod   time  total
--------------------- ------ ------ ------ ------ ------ ------ ------
blib/lib/Dumpvalue.pm   95.8   74.7   59.7  100.0   45.8  100.0   81.2
Total                   95.8   74.7   59.7  100.0   45.8  100.0   81.2
--------------------- ------ ------ ------ ------ ------ ------ ------

$ cat 8e07bd5a.coverage.txt
--------------------- ------ ------ ------ ------ ------ ------ ------
File                    stmt   bran   cond    sub    pod   time  total
--------------------- ------ ------ ------ ------ ------ ------ ------
blib/lib/Dumpvalue.pm   98.7   89.0   70.9  100.0   45.8  100.0   88.6
Total                   98.7   89.0   70.9  100.0   45.8  100.0   88.6
--------------------- ------ ------ ------ ------ ------ ------ ------