Migrate cluster module and adapt it

[georgiy-belyanin]

The original cluster module (tarantool/test/config-luatest/cluster.lua) has been moved to the current project and will be available as follows:

local t = require('luatest')
local cluster = t.cluster:new(...)

cluster:start()

It is used to simplify managing Tarantool clusters based on the provided configuration. The helper requires Tarantool 3.0.0 or newer. Otherwise cluster methods cause an error.

Original helper created by: [email protected]
Test author: [email protected]

Closes #368

[georgiy-belyanin]

The changes I've made to the original helper:

• g.cluster variable preserving the cluster is now g._cluster to avoid clashing with the module itself in this scenario:

local t = require('luatest')
-- If the created cluster is saved in g.cluster the clash occurs
local cluster = t.cluster.new(...)

• Now cluster.init/drop/clean hooks are registered automatically (I've dropped init() since it only had a stub in it).
• I've changed the wait_for_start code a bit since there is a problem with capturing upvalue luatest by server:exec.

-             server:exec(function()
-                 t.helpers.retrying({timeout = 60}, function()
-                     t.assert_equals(box.info.status, 'running')
-                 end)
+             helpers.retrying({timeout = 60}, function()
+                 assertions.assert_equals(server:eval('return box.info.status'),
+                                          'running')

• Some annoying renames local server -> local iserver in the helper, cluster -> c in the tests due to the linter settings.
• A little bit of extra docs for consistency.

[Totktonada]

I've no objections. Thanks for working on it!

Left a few remarks about consistency with other luatest APIs.

[Totktonada]

@georgiy-belyanin Thanks for the updates! I'm ready to merge it.

@locker Do you have any objections regarding luatest-consistent API?

Personally, I don't like all this quazi-oop, but it seems like this is the luatest way.

[locker]

@locker Do you have any objections regarding luatest-consistent API?

No.