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Allow to use UNITY build see https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html (Reduce compile time. Need cmake 3.16.0)

If the class is already exported from the library we cannot add the
.cpp file to the test again - MSVC then complains that the symbol is
exported twice (once from the test and once from the library that the
test is linked against)

Otherwise it appeared as a spurious line at the top left, and
prevented the "Expire read after" checkbox being clicked on.