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/4374.js

@@ -0,0 +1,32 @@
+describe("bug #4374: OAS3 parameters should be visibly validated in Try-It-Out", function () {
+    let mainPage
+    beforeEach(function (client, done) {
+        mainPage = client
+            .url("localhost:3230")
+            .page.main()
+
+        client.waitForElementVisible(".download-url-input:not([disabled])", 10000)
+            .pause(1000)
+            .clearValue(".download-url-input")
+            .setValue(".download-url-input", "/test-specs/bugs/4374.yaml")
+            .click("button.download-url-button")
+            .pause(1000)
+
+        done()
+    })
+    afterEach(function (client, done) {
+        done()
+    })
+    it("indicates an error when a required parameter is not selected", function (client) {
+        client.waitForElementVisible(".opblock-tag-section", 10000)
+            .assert.containsText(".opblock-summary-path span", "/pet/findByStatus")
+            .click(".opblock")
+            .waitForElementVisible(".opblock.is-open", 5000)
+            .click(".try-out__btn")
+            .click(".btn.execute")
+            .pause(100)
+            .assert.cssClassPresent(".parameters-col_description select", "invalid")
+
+        client.end()
+    })
+})