Add Info tests (#561)
Add Info tests (#561)

This patch will improve coverage of rminfo.c

* Before
  - Line coverage : 80.4 %
  - Functions : 87.2 %

* After
  - Line coverage : 95.1 %
  - Functions : 98.2 %
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick