Mock Debian OS in SystemD test

In e660355, we added an additional test for nonexistent services,
including a test for a Debian-specific code path that runs invoke-rc.d.

Because this code path is only invoked on Debian systems, this test
would regularly fail for any other platforms.

This commit mocks the os.family fact for the nonexistent services tests.

Author: mhashizume

spec/unit/provider/service/systemd_spec.rb

@@ -335,6 +335,7 @@
 
     it "should consider nonexistent services to be disabled" do
       provider = provider_class.new(Puppet::Type.type(:service).new(:name => 'doesnotexist'))
+      allow(Facter).to receive(:value).with('os.family').and_return('debian')
       expect(provider).to receive(:execute).with(['/bin/systemctl','is-enabled', '--', 'doesnotexist'], {:failonfail => false})
                             .and_return(Puppet::Util::Execution::ProcessOutput.new("", 1))
       expect(provider).to receive(:execute).with(["/usr/sbin/invoke-rc.d", "--quiet", "--query", "doesnotexist", "start"], {:failonfail => false})