Include CTest for BUILD_TESTING option

[scpeters]

The recent release of dartsim 6.14.0 is failing to build in homebrew-core due to an issue with fetching GoogleTest (see Homebrew/homebrew-core#175563 (comment)). My first thought to work around this was to build the homebrew formula with -DBUILD_TESTING=OFF, but that option is neither defined nor used to disable test compilation. This includes CTest to define the option and adds logic to skip building tests when BUILD_TESTING is false.

We could separately try to find the googletest homebrew formula instead of using FetchContent if desired, but I think BUILD_TESTING is a useful option anyway, so I would start with this.

Before creating a pull request

• Document new methods and classes
• Format new code files using ClangFormat by running make format
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve all the compile warnings

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change
• Add Python bindings for new methods and classes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.56%. Comparing base (47666d6) to head (b816c68).
Report is 19 commits behind head on release-6.14.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-6.14    #1819   +/-   ##
=============================================
  Coverage         61.56%   61.56%           
=============================================
  Files               490      490           
  Lines             46021    46021           
=============================================
  Hits              28334    28334           
  Misses            17687    17687           

[jslee02]

Good to know that Homebrew doesn't allow using FetchContent. Thanks!