build: add prefix to ENABLE_TESTS to avoid clashing

[tearfur]

Avoid clashing with the outer project, when this project is used as add_subdirectory(wide-integer) and the outer project defines ENABLE_TEST as a cache variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (f236df9) to head (4b2258a).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #471    +/-   ##
========================================
  Coverage   100.0%   100.0%            
========================================
  Files          41       41            
  Lines        3893     3772   -121     
  Branches      320      320            
========================================
- Hits         3893     3772   -121     

see 22 files with indirect coverage changes

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[ckormanyos]

Thanks @tearfur. Merged