test: fix flakiness in app-luatest/http_client_test.lua

The problem could be easily reproduced with following command line:
./test/test-run.py
	$(yes app-luatest/http_client_test.lua | head -n 1000).

Before this commit we did a socket binding to know a free network port,
then close a socket and started httpd.py on that network port. However
it was not reliable and even with socket options SO_REUSE_PORT start of
httpd.py has failed. With proposed patch schema is changed: we start
httpd.py and pass only a socket family (AF_INET for TCP connection and
AF_UNIX for connection via Unix socket) and then reading output from a
process. Successfully started httpd.py prints a path to a Unix socket or
a pair of IP address and network port split with ":".

With proposed patch test has passed 1000 times without any problems.
Tests previously marked as "fragile" are passed too:

./test/test-run.py --builddir=$(pwd)/build box-tap/net.box.test.lua \
	box-tap/cfg.test.lua box-tap/session.storage.test.lua \
	box-tap/session.test.lua app-tap/tarantoolctl.test.lua \
	app-tap/debug.test.lua app-tap/inspector.test.lua \
	app-tap/logger.test.lua app-tap/transitive1.test.lua \
	app-tap/csv.test.lua app-luatest/http_client_test.lua

P.S. The problem with "fragile" tests is that rerunning hides other
problems. [1] is about "Address already in use" and [2] is about hangs
in test. I made a pull request with changes in http client module and
triggered CI run. Job has been passed, but in log [3] I see three test
restarts due to fails in http_client test related to my changes.

1. https://github.com/tarantool/tarantool-qa/issues/186
2. https://github.com/tarantool/tarantool-qa/issues/31
3. https://github.com/tarantool/tarantool/runs/7726358823?check_suite_focus=true

Closes https://github.com/tarantool/tarantool-qa/issues/186
Closes https://github.com/tarantool/tarantool-qa/issues/31

NO_CHANGELOG=testing
NO_DOC=testing
NO_TEST=testing