Fix cmake error: Mixed styles of target_link_libraries signatures

Fix cmake error: Mixed styles of target_link_libraries signatures

Plain and keyword target_link_libraries signatures cannot be mixed.

Complete error message:

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