Use BUILD_TESTING as the default value for Beast_BUILD_TESTS.

[JohannesWilde]

This is the variable most other boost libraries use for initial selection of whether to build their tests [or they default to OFF].

To me this makes usage as a third_party, where I don't necessarily want to run all tests with every build, more consistent.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.03%. Comparing base (075db2f) to head (1fa295b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2828      +/-   ##
===========================================
- Coverage    93.06%   93.03%   -0.03%     
===========================================
  Files          178      178              
  Lines        13687    13687              
===========================================
- Hits         12738    12734       -4     
- Misses         949      953       +4     

see 1 file with indirect coverage changes

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