Add missing ctest symlink in PATH directory

[michaelbprice]

Tools that expect cmake to be on the PATH will likely expect ctest to also be on the PATH, such as the Microsoft's Visual Studio Code extension for CMake.

[bamurtaugh]

Thanks for opening @michaelbprice!

Our team has been actively focused on an updated plan for community contributions and this repo moving forward, which we've now outlined in this issue: #1589. This includes moving to a couple new repos for images (https://github.com/devcontainers/images) and Features (https://github.com/devcontainers/features), and we anticipate to have a similar repo and process for templates/definitions.

If it's possible to add these as an update to the C++ image or Feature, please feel free to open a PR in the new repos. Otherwise, we'll keep everyone updated (likely via another issue in this repo or comment on #1589) when our new templates repo is available.

Please let me know if you have any questions, thank you!