Allow running handler serverless for testing

[PedroEdiaz]

This commit allow for serverless testing, this is a code snippet on how to use it:

import "core:testing"
import http "."

@(test)
test_handler :: proc(t: ^testing.T) {
	http.td.state=.Testing

	req : http.Request
	res : http.Response

	handler(&req, &res)

	testing.expect( t, res.status==.OK )
}

The testing report leaks but I'm not sure if those are from this pull request

        +++ leak       448B @ 0x7FA2B31210C0 [headers.odin:37:headers_set_unsafe()]
        +++ leak       113B @ 0x7FA2B3121288 [response.odin:227:_response_write_heading()]