cmake: build example as part of tests

[junghans]

No description provided.

[codecov]

Codecov Report

Merging #61 (4def2b2) into master (8e788d4) will decrease coverage by 1.67%.
The diff coverage is n/a.

❗ Current head 4def2b2 differs from pull request most recent head ef5d774. Consider uploading reports for the commit ef5d774 to get more accurate results

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   89.81%   88.14%   -1.67%     
==========================================
  Files          19       20       +1     
  Lines        1953     1990      +37     
==========================================
  Hits         1754     1754              
- Misses        199      236      +37     

see 1 file with indirect coverage changes

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[junghans]

@robashaw

18/18 Test #18: example ..........................Subprocess aborted***Exception:   0.19 sec
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid

No idea why the example core bumps.