The test as added in 79ec651a28 had no
effect: first because ‘guix time-machine --commit=X’, not followed by a
command, does nothing, and second because the “! COMMAND” shell stanza
does not have the desired effect (see <https://issues.guix.gnu.org/62406>).
This change rewrites the test to make it effective.
* tests/guix-time-machine.sh: Rewrite.
Change-Id: Ib44a11331c8625e346139a236cffa699cdbd02f2
