[test] Silence gcc 7.4 warning [NFC]

Without this gcc 7.4.0 complains with
 ../unittests/Analysis/ValueTrackingTest.cpp:937:66: error: ISO C++11 requires at least one argument for the "..." in a variadic macro [-Werror]
                          ::testing::ValuesIn(IsBytewiseValueTests));
                                                                   ^

llvm-svn: 365738