When we call add_test() from each test's CMakeLists.txt, we specify the
working directory to be that of the CMakeList.txt file itself. Create a
property to store that directory and reference it when run-lagom-target
is invoked by serenity.sh.
Note that for non-test Lagom targets which do not set this property, the
working directory will be Build/lagom (because the property will be the
empty string, which CMake resolves relative to the current build path).
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