Add lexical-cast to test-tracking.el

Buttercup 1.34 requires lexical-cast: t in all files defining
buttercup test suites for Emacs >= 29.
It was already documented as a requirement, but was only actually
required for some specific functionality.  The introduction of
Oclosures for Emacs versions that support them made it a hard
requirement.

tests/test-tracking.el

@@ -1,4 +1,4 @@
-;;; Automated tests for tracking.el
+;;; test-tracking.el --- Automated tests for tracking.el -*- lexical-binding: t; -*-
 
 (require 'tracking)