Add Montage tests (#562)
Add Montage tests (#562)

This patch will improve coverage of rmmontage.c

* Before
  - Line coverage : 78.2 %
  - Functions : 85.7 %

* After
  - Line coverage : 97.8 %
  - Functions : 100.0 %
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick