github-ci: use vardir option in tests runs

Got warning message:

  [014] WARGING: unix socket's "/home/ubuntu/actions-runner/_work/tarantool/tarantool/test/var/014_box/gh-5422-broken_snapshot.socket-iproto" path has length 108 symbols that is longer than 107. That likely will cause failing of tests.

It caused the following fail:

  [038] Starting instance autobootstrap_guest1...
  [038] Start failed: builtin/box/console.lua:865: failed to create server unix/:/home/ubuntu/actions-runner/_work/tarantool/tarantool/test/var/038_replication/autobootstrap_guest1.socket-admin: No buffer space available

To avoid of it use vardir option in tests runs to decrease paths length.

Closes tarantool/tarantool-qa#104