test/unittest/CMakeLists.txt: balance if()/endif() guards

[trofi]

Without the change cmake warns about mismatching guards as:

CMake Warning (dev) in test/unittest/CMakeLists.txt:
  A logical block opening on the line
    /build/rapidjson/test/unittest/CMakeLists.txt:83 (if)
  closes on the line
    /build/rapidjson/test/unittest/CMakeLists.txt:95 (endif)
  with mis-matching arguments.

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