Refactor luatest test related stuff

- Remove test/luatest_helpers.lua
- Move `default_cfg`, `env_cfg`, `box_cfg` functions from
  test/luatest_helpers.lua to test/instances/default.lua
- Move `instance_uri` function from test/luatest_helpers.lua
  to test/luatest_helpers/server.lua and rename it into
  `build_instance_uri`
- Break down one test into four smaller ones

test/instances/default.lua

@@ -1,19 +1,44 @@
 #!/usr/bin/env tarantool
 
-local helpers = require('test.luatest_helpers')
+local fun = require('fun')
+local json = require('json')
 
-box.cfg(helpers.box_cfg())
+local function default_cfg()
+    return {
+        work_dir = os.getenv('TARANTOOL_WORKDIR'),
+        listen = os.getenv('TARANTOOL_LISTEN'),
+        log = ('%s/%s.log'):format(os.getenv('TARANTOOL_WORKDIR'),
+            os.getenv('TARANTOOL_ALIAS')),
+    }
+end
+
+local function env_cfg()
+    local cfg = os.getenv('TARANTOOL_BOX_CFG')
+    if cfg == nil then
+        return {}
+    end
+
+    local res = json.decode(cfg)
+    assert(type(res) == 'table')
+    return res
+end
+
+-- Create a table for box.cfg from values passed while server initialization and
+-- the given argument.
+local function box_cfg(cfg)
+    return fun.chain(default_cfg(), env_cfg(), cfg or {}):tomap()
+end
+
+box.cfg(box_cfg())
 box.schema.user.grant('guest', 'super', nil, nil, {if_not_exists = true})
 
--- luatest_helpers.Server:start() unblocks only when this variable
--- becomes true.
+-- The Server:start function unblocks only when this variable becomes true.
 --
 -- Set it when the instance is fully operable:
+--   * The server listens for requests.
+--   * The database is bootstrapped.
+--   * Permissions are granted.
 --
--- * The server listens for requests.
--- * The database is bootstrapped.
--- * Permissions are granted.
---
--- Use luatest_helpers.Server:start({wait_for_readiness = false})
--- to don't wait for setting of this variable.
+-- Use server:start({wait_for_readiness = false}) to not wait for setting this
+-- variable.
 _G.ready = true

test/luatest_helpers/server.lua

@@ -34,9 +34,13 @@ Server.constructor_checks = fun.chain(Server.constructor_checks, {
     engine = '?string',
 }):tomap()
 
-function Server:initialize()
-    local vardir = fio.abspath(os.getenv('VARDIR') or 'test/var')
+Server.socketdir = fio.abspath(os.getenv('VARDIR') or 'test/var')
+
+function Server.build_instance_uri(alias)
+    return ('%s/%s.iproto'):format(Server.socketdir, alias)
+end
 
+function Server:initialize()
     if self.id == nil then
         local random = digest.urandom(9)
         self.id = digest.base64_encode(random, {urlsafe = true})
@@ -45,13 +49,13 @@ function Server:initialize()
         self.command = 'test/instances/default.lua'
     end
     if self.workdir == nil then
-        self.workdir = ('%s/%s-%s'):format(vardir, self.alias, self.id)
+        self.workdir = ('%s/%s-%s'):format(self.socketdir, self.alias, self.id)
         fio.rmtree(self.workdir)
         fio.mktree(self.workdir)
     end
     if self.net_box_port == nil and self.net_box_uri == nil then
-        self.net_box_uri = ('%s/%s.iproto'):format(vardir, self.alias)
-        fio.mktree(vardir)
+        self.net_box_uri = self.build_instance_uri(self.alias)
+        fio.mktree(self.socketdir)
     end
 
     -- AFAIU, the inner getmetatable() returns our helpers.Server

test/test-luatest/smoke_check_test.lua

@@ -1,10 +1,11 @@
 local t = require('luatest')
-local g = t.group()
 
-local luatest_helpers = require('test.luatest_helpers')
+local server = require('test.luatest_helpers.server')
+
+local g = t.group()
 
 g.before_all = function()
-    g.server = luatest_helpers.Server:new({
+    g.server = server:new({
         alias = 'my_server',
         env = {MY_ENV_VAR = 'test_value'},
         box_cfg = {memtx_memory = 100 * 1024 ^ 2},
@@ -16,20 +17,22 @@ g.after_all = function()
     g.server:stop()
 end
 
-g.test_smoke = function()
-    -- The server is started and operable.
+g.test_server_is_started_and_operable = function()
     local res = g.server:eval('return 42')
     t.assert_equals(res, 42)
+end
 
-    -- The database is bootstrapped and accessible.
-    local res = g.server:eval('return box.info.status')
+g.test_database_is_bootstrapped_and_accessible = function()
+    local res = g.server:exec(function() return box.info.status end)
     t.assert_equals(res, 'running')
+end
 
-    -- The environment variable is passed.
-    local res = g.server:eval('return os.getenv("MY_ENV_VAR")')
+g.test_environment_variable_is_passed = function()
+    local res = g.server:exec(function() return os.getenv('MY_ENV_VAR') end)
     t.assert_equals(res, 'test_value')
+end
 
-    -- The box.cfg() values are passed as well.
-    local res = g.server:eval('return box.cfg.memtx_memory')
+g.test_box_cfg_values_are_passed = function()
+    local res = g.server:exec(function() return box.cfg.memtx_memory end)
     t.assert_equals(res, 100 * 1024 ^ 2)
 end