[bug#27599] [PATCH 1/2] gnu: Add cmdtest.

[Arun Isaac <arunisaac@systemreboot.net>]

I've figured out all the problems with cmdtest's tests. Please find
below the arguments I used:

(arguments
     `(#:python ,python-2
       #:phases
       (modify-phases %standard-phases
         ;; check phase needs to be run before the build phase. If not, the
         ;; coverage test runner looks for tests for the built source files,
         ;; and fails.
         (delete 'check)
         (add-before 'build 'check
           (lambda _
             (substitute* "yarn"
               (("/bin/sh") (which "sh")))
             ;; yarn uses python2-ttystatus to print messages.
             ;; python2-ttystatus requires /dev/tty which is not present in
             ;; the build environment. Hence assuming-failure test fails.
             (delete-file "yarn.tests/assuming-failure.script")
             (delete-file "yarn.tests/assuming-failure.stdout")
             (zero? (system* "python" "setup.py" "check")))))))

Do complete the patchset, with changes mentioned earlier, and
push. Thanks!