Ts all the tests (#1998)

* Convert tests to TypeScript

* Add types for tests

* Rename files to TS

* Fix up test imports

* Fix TSC errors

* Liiiint

* Add types to util method signatures

* Some more types

test-mocha/common/datetime/format_date.ts

@@ -0,0 +1,17 @@
+import { assert } from "chai";
+
+import formatDate from "../../../src/common/datetime/format_date";
+
+describe("formatDate", () => {
+  const dateObj = new Date(2017, 10, 18, 11, 12, 13, 1400);
+
+  it("Formats English dates", () => {
+    assert.strictEqual(formatDate(dateObj, "en"), "November 18, 2017");
+  });
+
+  // Node only contains intl support for english formats. This test at least ensures
+  // the fallback to a different locale
+  it("Formats other dates", () => {
+    assert.strictEqual(formatDate(dateObj, "fr"), "2017 M11 18");
+  });
+});