Provide helpers for luatest

The commit includes a few helpers files for luatest infrastructure.
'Server' based on luatest Server with additional functionality like
waiting Server for start, create Server's own env including box.cfg
provided by test.

Servers could be built and added in a cluster (cluster.lua file).

Asserts.lua is additional checks, which are useful for many tests.

Helpers are supportive function, could be also used for many tests.

Part of tarantool/test-run#304