Add tests for server's stop

Tests were added in attempt to reproduce #76.

test/integration/http_server_action_test.lua

@@ -0,0 +1,79 @@
+local fio = require('fio')
+local fiber = require('fiber')
+local socket = require('socket')
+local t = require('luatest')
+local http_client = require('http.client')
+local http_server = require('http.server')
+
+local helpers = require('test.helpers')
+
+local g = t.group()
+
+g.before_test('test_stop_basic', function()
+    g.httpd = helpers.cfgserv()
+    g.httpd:start()
+end)
+
+g.before_test('test_stop_with_open_connection', function()
+    g.httpd = helpers.cfgserv()
+    g.httpd:start()
+end)
+
+g.after_test('test_stop_basic', function()
+    helpers.retrying({}, function()
+        local conn = socket.tcp_connect(helpers.base_host, helpers.base_port)
+        return conn ~= nil
+    end)
+    fio.rmdir(helpers.path)
+end)
+
+g.after_test('test_stop_with_open_connection', function()
+    helpers.retrying({}, function()
+        local r = http_client.request('GET', helpers.base_uri)
+        return r == nil
+    end)
+    fio.rmdir(helpers.path)
+end)
+
+g.test_stop_basic = function()
+    local httpd = g.httpd
+    httpd:route({
+        method = 'GET',
+        path = '/test/json',
+    }, 'test#json')
+
+    local resp = httpd:stop()
+    t.assert_not_equals(resp, nil)
+    t.assert_type(resp, 'table')
+end
+
+g.test_stop_with_open_connection = function()
+    local httpd = g.httpd
+    httpd:route({
+        method = 'GET',
+        path = '/test/json',
+    }, 'test#json')
+
+    -- Open connection to the web server.
+    local resp = http_client.get(helpers.base_uri)
+    t.assert_not_equals(resp, nil)
+
+    -- Stop server.
+    httpd:stop()
+    t.assert_not_equals(resp, nil)
+    t.assert_type(resp, 'table')
+end
+
+g.test_stop_with_custom_handler = function()
+    local httpd = http_server.new('0.0.0.0', 8080)
+    httpd:route({
+        path = '/'
+    }, function ()
+        return {
+            status = 200, body = 'hello'
+        }
+    end)
+    httpd:start()
+    fiber.sleep(0.1)
+    httpd:stop()
+end