Remove legacy SERVER_TEST compiler flag from cmake.

[karthyuom]

This PR is to cleanup the SERVER_TEST compiler flag from cmake compile definitions, as it is no longer required in the new unit test framework (see #428).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.88%. Comparing base (0a89571) to head (d13adf3).
Report is 4 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1530      +/-   ##
============================================
+ Coverage     70.87%   70.88%   +0.01%     
============================================
  Files           120      120              
  Lines         64959    64959              
============================================
+ Hits          46041    46048       +7     
+ Misses        18918    18911       -7     

see 11 files with indirect coverage changes