[test] Avoid P0138R2, direct-list-init of fixed enums from integers, part 1/3.

[test] Avoid P0138R2, direct-list-init of fixed enums from integers, part 1/3.

This C++17 Core Language feature isn't necessary when testing std::byte.
It's a minor convenience, but it limits test coverage to very new compilers.

This part changes the code.

Fixes D32386.

llvm-svn: 302944