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

Missing IMPORTED_LOCATION_<CONFIG>property for shared-library import-targets (see zeromq/libzmq#2985)

Fixes for mingw cross-compilation