Add syntax sugar for more convenient logging

[ochaplashkin]

You can use luatest.log for logging. This is convenient in case of debugging in several places at the same time:

    local luatest = require('luatest')
    
    luatest.log('outside')
    g.test_foo = function()
        luatest.log('inside')
        g.server:exec(function() luatest.log('hi!') end)
    end

    I> outside
    I> inside
    I> hi!

The pretty conversion of arguments of any type will be performed automatically:

    luatest.log('My structure is %s', {a = 1, b = 2, c = {cc = 1}})
    I> My structure is {a = 1, b = 2, c = {cc = 1}}

Closes #326