Fix reserved cgroup systemd

Fix an issue in which, when trying to specify the `--kube-reserved-cgroup`
(or `--system-reserved-cgroup`) with `--cgroup-driver=systemd`, we will
not properly convert the `systemd` cgroup name into the internal cgroup
name that k8s expects. Without this change, specifying
`--kube-reserved-cgroup=/test.slice --cgroup-driver=systemd` will fail,
and only `--kube-reserved-cgroup=/test --crgroup-driver=systemd` will succeed,
even if the actual cgroup existing on the host is `/test.slice`.

Additionally, add light unit testing of our process from converting to a
systemd cgroup name to kubernetes internal cgroup name.

pkg/kubelet/cm/cgroup_manager_linux_test.go

@@ -146,3 +146,25 @@ func TestCgroupNameToCgroupfs(t *testing.T) {
 		}
 	}
 }
+
+func TestParseSystemdToCgroupName(t *testing.T) {
+	testCases := []struct {
+		input    string
+		expected CgroupName
+	}{
+		{
+			input:    "/test",
+			expected: []string{"test"},
+		},
+		{
+			input:    "/test.slice",
+			expected: []string{"test"},
+		},
+	}
+
+	for _, testCase := range testCases {
+		if actual := ParseSystemdToCgroupName(testCase.input); !reflect.DeepEqual(actual, testCase.expected) {
+			t.Errorf("Unexpected result, input: %v, expected: %v, actual: %v", testCase.input, testCase.expected, actual)
+		}
+	}
+}