Start the server without a pre-installed `box.cfg` configuration

[ochaplashkin]

When we start the server as follows:

local s = Server:new()
s:start()

The instance is already configured with some parameters:

local function default_cfg()
    return {
        work_dir = os.getenv('TARANTOOL_WORKDIR'),
        listen = os.getenv('TARANTOOL_LISTEN'),
        log = fio.pathjoin(
            os.getenv('TARANTOOL_WORKDIR'),
            os.getenv('TARANTOOL_ALIAS') .. '.log'
        ),
    }
end

This can be inconvenient if you need to test the startup process.

[sergepetrenko]

The ability to test an unconfigured node is getting more important since we are moving towards disallowing box.cfg calls on the default test runner (tarantool/tarantool#8709).

For example, some of the replication tests need an unconfigured instance to check failing box.cfg{} calls for non-dynamic options:

https://github.com/tarantool/tarantool/blob/606e50c4d56bb76714dc41339102b32230fd751e/test/replication-luatest/linearizable_test.lua#L295-L298

or

https://github.com/tarantool/tarantool/blob/606e50c4d56bb76714dc41339102b32230fd751e/test/replication-luatest/bootstrap_strategy_test.lua#L443-L448

Currently it's really hard to rework such tests by reusing existing server instance. Just look at commit tarantool/tarantool@9b2bee3447fc6a1f1a47a7f5a00c99e8f0bdb145

Another example where we rely on an unconfigured instance (testing what box.ctl.promote() call produces on an unconfigured box):

https://github.com/tarantool/tarantool/blob/606e50c4d56bb76714dc41339102b32230fd751e/test/replication-luatest/gh_6033_box_promote_demote_test.lua#L121-L130