Test that can close an already closed review

Add a test to phlcon_differential__t to test the assumption that we can
close an already closed review without error.

Test Plan: $ ./precommit.sh
Arcyd: refactor abdt_conduitmock: no magic strings

Oddly abdt_conduitmock was assigning string directly to the 'status'
attribute.  Use methods instead so errors are easier to detect.

Test Plan: $ ./precommit.sh
Arcyd: fix example in help string