Make unittests include path relative

This change is relevant when embedding the llvm cmake project into
another project.  It should not change the build behavior of a normal
llvm build.

In the case where llvm is embedded as a cmake subproject,
CMAKE_SOURCE_DIR does not point to the expected directory and building
the tests fails.
Using CMAKE_CURRENT_SOURCE_DIR fixes this problem, as it will always
point to the same directory.

Differential Revision: https://reviews.llvm.org/D73466