Problem:  global random init/deinit breaks existing applications

Solution: restrict it only to the original issue #2632, Tweetnacl on
*NIX when using /dev/urandom, ie: without the new Linux getrandom()
syscall.

Existing applications might use atexit to register cleanup functions
(like CZMQ does), and the current change as-is imposes an ordering
that did not exist before - the context MUST be created BEFORE
registering the cleanup with atexit. This is a backward incompatible
change that is reported to cause aborts in some applications.

Although libsodium's documentation says that its initialisation APIs
is not thread-safe, nobody has ever reported an issue with it, so
avoiding the global init/deinit in the libsodium case is the less
risky option we have.

Tweetnacl users on Windows and on Linux with getrandom (glibc 2.25 and
Linux kernel 3.17) are not affected by the original issue.

Fixes #2991

Migrate further tests to unity

Solution: migrate to unity, merge into test_radio_dish, and split up test cases

Solution: migrate to unity

Solution: migrate to unity, and split test cases

Solution: migrate to unity, and split test cases

Solution: Moved bind_loopback_ipv4 to testutil_unity to allow for reuse

Fix for #2997

Problem: Documentation missing for ZMQ_METADATA socket option

Migrate further tests to unity, and split up into individual test cases

Solution: split test case

Solution: split test case

Solution: split test case

Solution: migrate to unity, and split test cases

Solution: migrate to unity, and split test cases

Solution: migrate to unity, and reduce code duplication

Problem: no support for ZMTP 3.1 application metadata

Solution: add appropriate definitions to build

Solution: migrate to unit and split up test cases

Migrate more tests to unity

Solution: add unity dependencies/flags

Solution: migrate to unity

Solution: split up test case

Solution: migrate to unity

Problem: test assertion failures do not clean up properly

Lets the application set per-connection metadata.
Metadata is specified as "X-key:value" and set using zmq_setsockopt, eg:
zmq_setsockopt (s, ZMQ_METADATA, "X-key:value", 11);

The peer can then obtain the metadata from a received message:
char *data = zmq_msg_gets(msg, "X-key");

Solution: add guidelines to tests/README.md

Solution: provide test_context and manage sockets of test_context via setup and teardown

Migrate further tests (test_hwm, test_router_mandatory) to unity

Solution: removed

Solution: fixed description for building tests, added references to unity and unittests

Solution: migrate to unity