gutils test: Don't assume that every Unix OS implements os-release(5)
os-release(5) is widely implemented on Linux, but not necessarily ubiquitous: unusual or minimal Linux distributions might not have it. It could in principle be implemented by any other Unix OS, but in practice this has not yet happened.
Closes: #1906 (closed) Fixes: 349318e8 "gutils: Add g_get_os_info()"
/cc @robert.ancell