The previous code worked "by chance": 'start' from (shepherd service)
happened to be in scope because the marionette REPL is created by a mere
'primitive-fork', and 'start' happened to kinda work.
* gnu/tests/base.scm (run-basic-test): Use 'start-service' from (gnu
services herd), not 'start' from (shepherd service), which is not
supposed to work.
* gnu/tests/install.scm (run-install): Likewise.