Added artifacts saver to all gitlab-ci jobs with testing.

Gitlab-ci jobs saves its results files in the following paths:

  1. base jobs for testing different features:
    - test/var/artifacts

  2. OSX jobs:
    - ${OSX_VARDIR}/artifacts

  3. pack/deploy jobs:
    - build/usr/src/*/tarantool-*/test/var/artifacts

  4. VBOX jobs (freebsd_12) on virtual host:
    - ~/tarantool/test/var/artifacts

In gitlab-ci configuration added 'after_script' section with script
which collects from different test places 'artifacts' directories
created by test-run tool. It saves 'artifacts' directories as root
path in artifacts packages. User will be able to download these
packages using gitlab-ci GUI either API.

Additionally added OSX_VARDIR environment variable to be able to
setup common path for artifacts and OSX shell scripts options.

  OSX_VARDIR: /tmp/tnt

Part of #5050