cpp,tests: Add test runner for doing a WKD lookup without import

* lang/cpp/tests/Makefile.am (run_wkdlookup_SOURCES, programs_unix):
New.
(noinst_PROGRAMS): Add $(programs_unix).
* lang/cpp/tests/run-wkdlookup.cpp: New.

This test runner is a prototype for doing a WKD lookup without implicit
import of the found key(s).

Building this test runner is disabled on Windows because it may not
work there. Feel free to enable it on Windows if the test runner works
there.

• GnuPG-bug-id: T5728