test: fix import and usage luatest module

After adding the autorequiring luatest [1,2], there is no need to use
the following approach now:

```
local t = require('luatest')
local g = t.group()

server:exec(function()
    local t = require('luatest') -- duplicate
    t.assert(...)
end)
```

Modern approach looks like:

```
local t = require('luatest')
local g = t.group()

-- `t` already available in the remote server
server:exec(function() t.assert(...) end)

-- also it works with any variable
local my_custom_t = require('luatest')

server:exec(function()
    my_custom_t.assert(...) -- already available
end)
```

[1] tarantool/luatest#277
[2] tarantool/luatest#289

Part of tarantool/luatest#233

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix