Added test package in nix flake to automatically run tests.

To be able to do such, it was required to add the vendor directory
to the git project.

vendor/deno.land/std@0.191.0/testing/_format.ts

@@ -0,0 +1,23 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+// This module is browser compatible.
+
+/**
+ * Converts the input into a string. Objects, Sets and Maps are sorted so as to
+ * make tests less flaky
+ * @param v Value to be formatted
+ */
+export function format(v: unknown): string {
+  // deno-lint-ignore no-explicit-any
+  const { Deno } = globalThis as any;
+  return typeof Deno?.inspect === "function"
+    ? Deno.inspect(v, {
+      depth: Infinity,
+      sorted: true,
+      trailingComma: true,
+      compact: false,
+      iterableLimit: Infinity,
+      // getters should be true in assertEquals.
+      getters: true,
+    })
+    : `"${String(v).replace(/(?=["\\])/g, "\\")}"`;
+}