Bootstrap email-editor unit tests

I also replaced using verify function by asserts, which will be removed in the higher version of phpunit.
[MAILPOET-6216]

packages/php/email-editor/tests/unit/_stubs.php

@@ -0,0 +1,14 @@
+<?php declare(strict_types = 1);
+// phpcs:ignoreFile - We want to allow multiple classes etc.
+
+// Dummy WP classes
+if (!class_exists(\WP_Theme_JSON::class)) {
+  class WP_Theme_JSON {
+    public function get_data() {
+      return [];
+    }
+    public function get_settings() {
+      return [];
+    }
+  }
+}