meta: introduce Cypress end-to-end testing (via #4827)

* `test/e2e` -> `test/e2e-selenium`

* add Cypress

* ESLint fixes

* MOAR cypress

* `integration` -> `tests`

* wire Cypress up to a hot e2e server

* linter fixes

* don't run in CI

test/e2e-selenium/scenarios/oas3/callbacks.js

@@ -0,0 +1,41 @@
+describe("render pet api container", function () {
+  let mainPage
+  let apiWrapper
+  beforeEach(function (client, done) {
+    mainPage = client
+    .url("localhost:3230")
+    .page.main()
+
+    client.waitForElementVisible(".download-url-input:not([disabled])", 5000)
+    .pause(5000)
+    .clearValue(".download-url-input")
+    .setValue(".download-url-input", "http://localhost:3230/test-specs/callbacks.openapi.yaml")
+    .click("button.download-url-button")
+    .pause(1000)
+
+    apiWrapper = mainPage.section.apiWrapper
+
+    done()
+  })
+  afterEach(function (client, done) {
+    done()
+  })
+  describe("POST /pet", () => {
+    it("should render a callback correctly", function (client) {
+      apiWrapper.waitForElementVisible("#operations-pet-addPet", 10000)
+      // Expand the operation
+      .click("#operations-pet-addPet")
+      .waitForElementVisible("#operations-pet-addPet > div:nth-child(2) > div", 5000)
+      // Switch to Callbacks tab
+      .click("#operations-pet-addPet div.tab-header > div.tab-item.false > h4 > span")
+      .waitForElementVisible("#operations-pet-addPet div.callbacks-container", 5000)
+      .assert.containsText("#operations--post_request_body__url > div > span.opblock-summary-path", "$request.body#/url")
+      .click("#operations--post_request_body__url")
+      .waitForElementVisible("#operations--post_request_body__url div.response-col_description__inner > div > div > p", 5000)
+      .assert.containsText("#operations--post_request_body__url div.response-col_description__inner > div > div > p", "webhook successfully processed and no retries will be performed")
+
+
+      client.end()
+    })
+  })
+})