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/topbar.js

@@ -0,0 +1,52 @@
+describe("initial render", function () {
+    let mainPage
+    describe("for topbar", function () {
+        let topbar
+        before(function (client, done) {
+        done()
+        })
+
+        after(function (client, done) {
+        client.end(function () {
+            done()
+        })
+        })
+
+        afterEach(function (client, done) {
+        done()
+        })
+
+        beforeEach(function (client, done) {
+        mainPage = client
+            .url("localhost:3230")
+            .page.main()
+
+        topbar = mainPage.section.topbar
+
+        client.waitForElementVisible(".download-url-input:not([disabled])", 10000)
+            .pause(5000)
+            .clearValue(".download-url-input")
+            .setValue(".download-url-input", "http://localhost:3230/test-specs/petstore.json")
+            .click("button.download-url-button")
+            .pause(1000)
+
+        done()
+        })
+
+        it("renders section", function (client) {
+        mainPage.expect.section("@topbar").to.be.visible
+        client.end()
+        })
+
+        it("renders input box", function (client) {
+        topbar.expect.element("@inputBox").to.be.visible
+        client.end()
+        })
+
+        it("renders explore button", function (client) {
+        topbar.expect.element("@btnExplore").to.be.visible
+
+        client.end()
+        })
+    })
+})