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/specs/features/allow-empty-value.openapi.yaml

@@ -0,0 +1,64 @@
+openapi: "3.0.0"
+
+paths:
+  /regularParams:
+    get:
+      parameters: 
+      - name: int
+        in: query
+        schema:
+          type: integer
+      - name: str
+        in: query
+        schema:
+          type: string
+      - name: num
+        in: query
+        schema:
+          type: number
+      - name: bool
+        in: query
+        schema:
+          type: boolean
+      - name: arr
+        in: query
+        schema:
+          type: array
+          items:
+            type: string
+      responses:
+        200:
+          description: ok
+  /emptyValueParams:
+    get:
+      parameters: 
+      - name: int
+        in: query
+        schema:
+          type: integer
+        allowEmptyValue: true 
+      - name: str
+        in: query
+        schema:
+          type: string
+        allowEmptyValue: true 
+      - name: num
+        in: query
+        schema:
+          type: number
+        allowEmptyValue: true 
+      - name: bool
+        in: query
+        schema:
+          type: boolean
+        allowEmptyValue: true 
+      - name: arr
+        in: query
+        schema:
+          type: array
+          items:
+            type: string
+        allowEmptyValue: true
+      responses:
+        200:
+          description: ok