Adds some extra test coverage

pkg/system/systemd_test.go

@@ -49,7 +49,7 @@ func (f *fakeDbusConn) RestartUnit(name string, mode string, ch chan<- string) (
 }
 
 func (f *fakeDbusConn) ListUnits() ([]dbus.UnitStatus, error) {
-	return f.unitStatuses, nil
+	return f.unitStatuses, f.errors["list"]
 }
 
 func TestEnsureRunning(t *testing.T) {
@@ -193,3 +193,48 @@ func TestContainerRuntime(t *testing.T) {
 		})
 	}
 }
+
+func TestContainerRuntimeErrors(t *testing.T) {
+	testCases := []struct {
+		desc         string
+		unitStatuses []dbus.UnitStatus
+		expected     error
+		error        error
+	}{
+		{
+			desc: "When there is a systemd error",
+			unitStatuses: []dbus.UnitStatus{
+				{
+					Name:      "docker.service",
+					LoadState: "loaded",
+				},
+			},
+			expected: errors.New("a systemd error"),
+			error:    errors.New("a systemd error"),
+		},
+		{
+			desc: "When no container runtime is loaded",
+			unitStatuses: []dbus.UnitStatus{
+				{
+					Name:      "containerd.service",
+					LoadState: "Not found",
+				},
+			},
+			expected: errors.New("couldn't work out what container runtime is installed"),
+		},
+	}
+	for _, tC := range testCases {
+		tC := tC
+		t.Run(tC.desc, func(t *testing.T) {
+			d := &fakeDbusConn{unitStatuses: tC.unitStatuses, errors: map[string]error{"list": tC.error}}
+			s := &system.Systemd{Conn: d}
+			_, err := s.ContainerRuntime()
+			if err == nil {
+				t.Errorf("Expected an error!")
+			}
+			if err.Error() != tC.expected.Error() {
+				t.Errorf("Expected error to be %v but was: %v", tC.expected, err)
+			}
+		})
+	}
+}