Add end-to-end test in tests/test_e2e.py

Introduce a test that starts with C++ source code and verifies that
Doxylink generates correct links in the HTML. This test makes it easy to
reproduce issues and confirm when they are fixed.

Change-Id: I2a1f5460bc81fc241ce42bab78b9425f7538f5be
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/sphinx-contrib/doxylink/+/386617