Added tests for mithril (#11)

I've used a `wait` in the tests because mithril renders on nextTick. I'm unsure if this is the right way to test it though.

mithril.test.js

@@ -0,0 +1,33 @@
+import 'jest-dom/extend-expect'
+import m from 'mithril'
+import {getQueriesForElement, fireEvent, wait} from 'dom-testing-library'
+
+const Counter = () => {
+  let count = 0
+  return {
+    view: () =>
+      m(
+        'button',
+        {
+          onclick: () => {
+            count++
+          },
+        },
+        count,
+      ),
+  }
+}
+
+// tests:
+test('counter increments', async () => {
+  const div = document.createElement('div')
+  m.mount(div, Counter)
+
+  const {getByText} = getQueriesForElement(div)
+  const counter = getByText('0')
+  fireEvent.click(counter)
+  await wait(() => expect(counter).toHaveTextContent('1'))
+
+  fireEvent.click(counter)
+  await wait(() => expect(counter).toHaveTextContent('2'))
+})

package.json

@@ -22,6 +22,7 @@
     "hyperapp": "^1.2.9",
     "jquery": "^3.3.1",
     "knockout": "^3.4.2",
+    "mithril": "^1.1.6",
     "preact": "^8.3.1",
     "react": "^16.5.1",
     "stimulus": "^1.1.0",