feat: migrate unit tests to Jest (#6353)

* config(jest): updated setup
* config(jest): update testMatch to include jsx files
* config(jest): add transformIgnorePatterns
* config(jest): update ignore files that do not work in jest yet
* config: add test:unit-jest to test script

* fix(jest): lint with eslint-plugin-jest

* refactor(jest): move unit test directory
* refactor(mocha): restore mocha tests that fail in jest

* docs(jest): update helpful scripts with test:unit-jest

test/unit/bugs/3279-empty-markdown-source.jsx

@@ -0,0 +1,35 @@
+import React from "react"
+import { render } from "enzyme"
+import Markdown from "components/providers/markdown"
+
+describe("UI-3279: Empty Markdown inputs causing bare `undefined` in output", function(){
+  it("should return no text for `null` as source input", function(){
+    let props = {
+      source: null
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+
+  it("should return no text for `undefined` as source input", function(){
+    let props = {
+      source: undefined
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+
+  it("should return no text for empty string as source input", function(){
+    let props = {
+      source: ""
+    }
+
+    let el = render(<Markdown {...props}/>)
+
+    expect(el.text()).toEqual("")
+  })
+})