Add new tests cases & test housekeeping

Summary:
related to T12015
- error.hpp covered
- assemble_op.hpp fully covered
- malformed_pubrel test is now functional
- attempt at using fixtures in malformed_packet.cpp test suite to
lower the amount of copy-paste code lines
- test coverage ~89%

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D27395

test/integration/executors.cpp

@@ -172,6 +172,7 @@ BOOST_AUTO_TEST_CASE(async_run) {
 						"t_1", "p_1", retain_e::no, {},
 						[&](error_code ec, reason_code rc, auto) {
 							BOOST_CHECK_EQUAL(ec, asio::error::operation_aborted);
+							BOOST_CHECK_EQUAL(rc, reason_codes::empty);
 							BOOST_CHECK(strand.running_in_this_thread());
 							++handlers_called;
 						}