Test List macros in C++ extensions

Test List macros in C++ extensions

All of these macros already work in C++ with Clang and GCC (the only
compilers we're currently testing C++ extension support for).  This
adds a regression test for them in our test C++ extension, so we can
safely change their implementation without accidentally breaking C++.
Some of the List macros didn't work in C++ in the past (see commit
d5ca15ee5), and this would have caught that.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg@mail.gmail.com