Cryptonight variant 2 support

Reference code: https://github.com/monero-project/monero/pull/4218

src/workers/MultiWorker.cpp

@@ -55,6 +55,7 @@ bool MultiWorker<N>::selfTest()
     if (m_thread->algorithm() == CRYPTONIGHT) {
         return verify(VARIANT_0,   test_output_v0)  &&
                verify(VARIANT_1,   test_output_v1)  &&
+               verify(VARIANT_2,   test_output_v2)  &&
                verify(VARIANT_XTL, test_output_xtl) &&
                verify(VARIANT_MSR, test_output_msr) &&
                verify(VARIANT_XAO, test_output_xao) &&