Add basic tests for chflags, mkdir, rcp, and rmdir

Add basic command line parsing test coverage for these utilities. The tests
were automatically generated based on their man pages. These tests can be
expanded by hand for more thorough coverage. The aim is to generate very
basic amount of test coverage for all the utilities in the base system.

Submitted by: shivansh
Reviewed by: asomers, brooks
MFC after: 3 weeks
Sponsored by: Google, Inc (GSoC 2017)
Differential Revision: https://reviews.freebsd.org/D12036