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/4587.js

@@ -0,0 +1,36 @@
+describe("bug #4587: clearing header param values", function () {
+  let mainPage
+  beforeEach(function (client, done) {
+    mainPage = client
+      .url("localhost:3230")
+      .page.main()
+
+    client.waitForElementVisible(".download-url-input:not([disabled])", 5000)
+      .pause(2000)
+      .clearValue(".download-url-input")
+      .setValue(".download-url-input", "http://localhost:3230/test-specs/bugs/4587.yaml")
+      .click("button.download-url-button")
+      .pause(1000)
+
+    done()
+  })
+
+  afterEach(function (client, done) {
+    done()
+  })
+
+  it("sets a required initial value based the first enum value", function (client) {
+    client.waitForElementVisible(".opblock-tag-section", 10000)
+    .click("#operations-sql-execSql")
+    .waitForElementVisible(".opblock.is-open", 5000)
+      .click("button.btn.try-out__btn")
+      .setValue(`tr[data-param-name="x-irest-conn"] input`, "hi")
+      .click("button.btn.execute")
+      .waitForElementVisible(".request-url", 2000)
+      .setValue(`tr[data-param-name="x-irest-conn"] input`, `\u0008\u0008\u0008`) // backspaces
+      .pause(900)
+      .click("button.btn.execute")
+      .expect.element("textarea.curl").text
+      .to.not.contain(`x-irest-conn`)
+  })
+})