TESTS: add a unit test runner in the Makefile

TESTS: add a unit test runner in the Makefile

`make unit-tests` would run shell scripts from tests/unit/

The run-unittests.sh script will look for any .sh in tests/unit/ and
will call it twice:

- first with the 'check' argument in order to decide if we should skip
  the test or not
- second to run the check

A simple test could be written this way:

	#!/bin/sh

	check() {
	       ${HAPROXY_PROGRAM} -cc 'feature(OPENSSL)'
	       command -v socat
	}

	run() {
		 ${HAPROXY_PROGRAM} -dI -f ${ROOTDIR}/examples/quick-test.cfg -c
	}

	case "$1" in
	       "check")
	               check
	       ;;
	       "run")
	               run
	       ;;
	esac

The tests *MUST* be written in POSIX shell in order to be portable, and
any special commands should be tested with `command -v` before using it.

Tests are run with `sh -e` so everything must be tested.