Rename BuildTests to JSON_BuildTests
Rename BuildTests to JSON_BuildTests

This avoids potential conflicts with other libraries when the library is
built from source and included in a bigger cmake build.

nlohmann/json,hunter-packages/json,hunter-packages/json,theodelrieu/json,kaidokert/json,theodelrieu/json,kaidokert/json,hunter-packages/json,nlohmann/json,kaidokert/json,nlohmann/json