[DRE-maint] Bug#797972: add sprockets tests for rails engines

[Pirate Praveen]

I have tried to implement it in gem2deb-testrunner (smoke-test branch
of gem2deb)

http://anonscm.debian.org/cgit/pkg-ruby-extras/gem2deb.git/commit/?h=smoke-test&id=c4ccfad2fc3962b2cecea96a51206e469d6a8853

Can someone review it? May be using sprockets directly is better, but
I could not get that to work. I thought a heavier test is better than
no test.