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/on-complete.js

@@ -0,0 +1,28 @@
+const expect = require("expect")
+
+describe("onComplete option", function () {
+  let mainPage
+  beforeEach(function (client, done) {
+    mainPage = client
+      .url("localhost:3230")
+      .page.main()
+
+    client.waitForElementVisible(".download-url-input:not([disabled])", 5000)
+      .pause(80)
+      .clearValue(".download-url-input")
+      .setValue(".download-url-input", "http://localhost:3230/test-specs/petstore.json")
+      .click("button.download-url-button")
+      .pause(1000)
+
+    done()
+  })
+
+  it("triggers the page-provided onComplete exactly 1 times", function (client, done) {
+    client.execute(function() {
+      return window.completeCount
+    }, [], (result) => {
+      expect(result.value).toEqual(1)
+      client.end()
+    })
+  })
+})