Fix test failure for missing _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS

This is a follow-up change to r331150. The CL moved the macro from individual
file to build file, but the macro is missed in a test config file.

https://reviews.llvm.org/D46385
Patch from Taiju Tsuiki <tzik@chromium.org>!

llvm-svn: 331450