Raise an error when non-array arguments passed to the server:exec

[ochaplashkin]

Due to the specifics of the unpack function in the server:exec() method an array structure check has been added. An error will be raised if the arguments are not an array.

Normal cases

-- some_test.lua
server:exec(function(a, b)
    -- `a` is 'a'
    -- `b` is 'b'
end, {'a', 'b'})

server:exec(function()
    -- nothing
end)

Number of items and structure length

Due to the specific and non-deterministic # method, we cannot use this table attribute.
These cases will be skipped and will not be checked.

-- some_test.lua
server:exec(function(a)
    -- nothing
end)

server:exec(function(a,b)
    -- `a` is 'a'
    -- `b` is nil
end, {'a'})

server:exec(function(a)
    -- `a` is 'a'
    --  the rest of the arguments are cut off
end, {'a', 'b'})

Corner case with shifting arguments

-- some_test.lua
server:exec(function(a,b,c)
    -- `a` is 'a'
    -- `b` is 'c' (!) shifted
    -- `c` is nil
end, {'a', b='b', 'c'})

Luatest will give an error:

bad argument #3 for exec at malformed_args_test.lua:66: an array is required"

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Resolves #230

[ochaplashkin]

@sergepetrenko , hi!
I updated the logic of the work and added an obvious error if there are problems with the types in the arguments.

Everything else (logging) remained unchanged.

[sergepetrenko]

Thanks for working on this!

Looks good. Only one note from me.

[ochaplashkin]

I don't really like the current version, but I don't know yet how to work more comfortably with the table structure
Moved from draft to review.

[ylobankov]

LGTM