tests: port remaining C unit tests to the C++ test harness

Convert acl, acl_literal, aead2022, base64, buffer, cache (legacy
reference), crypto, ipset, ipset_reference, json, netutils, ppbloom,
process, ref, ssurl and utils tests to test_helpers.hpp. ctest names are
unchanged; every assertion is kept. Build the ported sources with the
strict C++ warning set.

tests/test_cache_cpp.cpp

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-3.0-or-later */
 // Edge cases of the fixed-capacity cache (src/cache.cpp) beyond
-// test_cache.c, and proof that a full cache churns without allocating.
+// test_cache_reference_cpp.cpp, and proof that a full cache churns without allocating.
 #include <cerrno>
 #include <cstdint>
 #include <cstdio>