cli_test: print command on failure

[eseiler]

Something to consider.

https://github.com/seqan/raptor/pull/115/files#diff-08ef433a2d158ef40fe8b8f6f078edf7c4ceaab22f7f70d583765035195c2467

Personally, I like to just plug the command into the debugger. This is way easier if the command is just printed on failure, otherwise I always need to add debug output.