[CMake] add extension to fix CMP0115


CMake Warning (dev) at unittest/CMakeLists.txt:10 (ADD_LIBRARY):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.


Call Stack (most recent call first):
  unittest/CMakeLists.txt:33 (ADD_LIB_UNIT_TEST)
This warning is for project developers.  Use -Wno-dev to suppress it.
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