vweb: add an optional before_accept_loop/0 method, to make testing easier and more robust

[spytheman]

Allow for more robust testing using:

struct App {
	vweb.Context
mut:
	started chan bool
}

pub fn (mut app App) before_accept_loop() {
	app.started <- true
}

and later doing:

mut app := &App{}
spawn vweb.run(app, port)
_ := <-app.started

... without having to use polling or brittle time.sleep() calls.

See https://github.com/vlang/v/blob/43bc3069631a5b7b6433456d95cdae1326b4d1be/vlib/vweb/tests/vweb_should_listen_on_both_ipv4_and_ipv6_by_default_test.v

[enghitalo]

x.vweb will need/have something like that?

[spytheman]

x.vweb will need/have something like that?

Probably yes, but that will be another PR.