C++20 port 16/16: config parsing on arenas, unit tests in C++ (#3080)

* Rename test_jconf.c to test_jconf_cpp.cpp

* Parse the config file on arenas

read_jconf now reads the file and builds the JSON tree in a scratch arena
that is released before it returns, and copies the parsed strings into one
process-lifetime arena instead of a malloc per string. parse_addr and the
config parser share one address splitter.

Port test_jconf to C++ and cover every field copied out of the parse tree,
a re-read, and the densest file the size limit accepts.

* tests: rename remaining C unit tests to .cpp (no content changes)

* 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.

* Run clang-tidy on every ported test

The test regex skipped file names with digits (test_aead2022_cpp.cpp,
test_base64_cpp.cpp); include them, fix the two optional accesses it then
reports, and build test_jconf with the strict warning set.

* docs: per-connection RSS on Linux against the C relay

* test_jconf: check read_jconf results before using them

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>