VWR-24251: Fix -DLL_TESTS:BOOL=ON on standalone when Tut is installed in a non-standard directory.

If tut/tut.hpp isn't installed in a standard include directory all tests
fail because the found include directory for tut isn't passed to the compiler.

This patch fixes this by passing it.
Note that using include_directories() in a Find*.cmake file is bad practise.
The correct way is to set an include dir variable and call
include_directories() once. It certainly doesn't work for the tests anyway
because the tests are all over the place and include_directories is on a
per folder basis.  What is needed is to set it for each (test) target.

However, there is no TARGET_INCLUDE_DIRECTORIES. The closest thing that we
have is to set the COMPILE_FLAGS property for a target.

Fortunately, standalone is only used for linux, so we can just use
-I${TUT_INCLUDE_DIR} to get the effect we want.