test: fix OSX host setup

Fixed OSX host setup for Tarantool build:
- set brew installation from Homebrew repository instructions;
- set in use Python 2 latest commit from tapped local formula,
  since Python 2 is EOL, also removed extra pip installation with
  get-pip script, because tapped formula installs pip itself.
  python@2 was deleted from homebrew/core in commit 028f11f9e:
    python@2: delete (https://github.com/Homebrew/homebrew-core/issues/49796)
    EOL 1 January 2020.
  Tapped formula created from the latest formula before its removal:
    git -C "$(brew --repo homebrew/core)" show 028f11f9e^:Formula/python@2.rb
- added upgrade packages call to avoid of fails on already
  installed packages, but with previous version;
- fixed the gitlab-ci configuration for sudo on testing hosts and removed pip
  option '--user' to avoid of use the users paths with special setup for it.

Fixed OSX host setup for Tarantool test:
- set maximum processes limit value to 2500 for testing process;
- new Mac machines are going to be added into CI and usernames on them
  are long according to internal policies. It makes a home directory to
  be long too and so a path to a unix socket created during testing can
  be longer then UNIX_PATH_MAX=108 constant which is described as issue
    https://github.com/tarantool/tarantool/issues/4634
  To avoid of it the short working directory for testing set by option:
    --vardir /tmp/tnt