The `OnSystem.os_condition_met?` method only handles Linux and macOS
values for the current OS, so this fails when testing with a generic
OS. This shortcoming is only being surfaced now because there weren't
any tests for `OnSystem` before.
This addresses the issue by accounting for macOS values (`:macos` or
a symbol from `MacOSVersion::SYMBOLS`) and returning `false` for any
other values (`:linux`, `:generic`, etc.).