Move duplicated setInterval utility functions from the TCP networking tests to a separate namespace

[rdw-software]

There's at least five different copy/pasted instances of a setInterval function in the scenarios for TCP networking... like this:

-- Creating a simple setInterval wrapper
local uv = require("uv")
local function setInterval(interval, callback)
    local timer = uv.new_timer()
    timer:start(interval, interval, function()
        callback()
    end)
    return timer
end

They should likely be moved to a C_Timer API namespace, and properly tested. Or maybe just relying on libuv tests is enough?