Add a test for persist properties settings

public class SystemPropertiesTest extends TestCase {

    private static final String KEY = "sys.testkey";

    private static final String PERSIST_KEY = "persist.sys.testkey";

    @SmallTest

    public void testStressPersistPropertyConsistency() throws Exception {

        for (int i = 0; i < 100; ++i) {

            SystemProperties.set(PERSIST_KEY, Long.toString(i));

            long ret = SystemProperties.getLong(PERSIST_KEY, -1);

            assertEquals(i, ret);

        }

    }

    @SmallTest

    public void testLongSequencialProperties() throws Exception {

    public void testStressMemoryPropertyConsistency() throws Exception {

        for (int i = 0; i < 100; ++i) {

            SystemProperties.set(KEY, Long.toString(i));

            long ret = SystemProperties.getLong(KEY, -1);