Fix cmake error: Mixed styles of target_link_libraries signatures

Plain and keyword target_link_libraries signatures cannot be mixed.

Complete error message:

------------------->8---------------------------8<-------------------------- CMake Error at src/testutils/CMakeLists.txt:76 (target_link_libraries): The keyword signature for target_link_libraries has already been used with the target "testutils". All uses of target_link_libraries with a target must be either all-keyword or all-plain.

The uses of the keyword signature are here:

• src/testutils/CMakeLists.txt:72 (target_link_libraries)

-- Configuring incomplete, errors occurred! ------------------->8---------------------------8<--------------------------

See: https://cmake.org/cmake/help/v3.0/policy/CMP0023.html