clang warns about throwing from RAPIDJSON_NOEXCEPT_ASSERT() in a nothrow
context.
If RAPIDJSON_ASSERT() throws it can never be used for _NOEXCEPT_ASSERT(),
so use C assert() instead.
Finally (and originally), since RAPIDJSON_ASSERT() in "unittest.h" throws,
make it define RAPIDJSON_ASSERT_THROWS for RAPIDJSON_NOEXCEPT_ASSERT() to
now do the right thing.