Adding -in and -nin flags to CommandTest

[ibnc]

• -in looks to see if a string is in the output of a command
• -nin looks to verify that a string is not in the output of a command

@barrowkwan These changes need to be made after https://github.com/gocd/gocd/pull/3374 is merged into the main gocd repository.

I'm unsure if there are tests I need to update?? I'm not familiar with golang, and I didn't see any files that looked like tests