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/bugs/4445.js

@@ -0,0 +1,32 @@
+describe("bug #4445: callback-via-$ref rendering", function () {
+    let mainPage
+    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/bugs/4445.yaml")
+            .click("button.download-url-button")
+            .pause(1000)
+
+        done()
+    })
+    afterEach(function (client, done) {
+        done()
+    })
+    it("expands an operation that has a visible callback", function (client) {
+        client.waitForElementVisible(".opblock-tag-section", 10000)
+            .click(".opblock")
+            .waitForElementVisible(".opblock-body", 5000)
+            .click(".opblock-section-header > .tab-header > div.tab-item:nth-of-type(2)")
+            .waitForElementVisible(".callbacks-container .opblock", 5000)
+            .click(".callbacks-container .opblock")
+            .waitForElementVisible(".callbacks-container .opblock-body", 5000)
+            .assert.containsText(".callbacks-container .opblock-description-wrapper:nth-of-type(2) p", "subscription payload")
+
+        client.end()
+    })
+})