[CMake] Use CMake's default RPATH for the unit tests

[CMake] Use CMake's default RPATH for the unit tests

In the top-level CMakeLists.txt, we set CMAKE_BUILD_WITH_INSTALL_RPATH to ON,
and then for the unit tests we set it to <test>/../../lib. This works for tests
that live in unittest/<whatever>, but not for those that live in subdirectories
e.g. unittest/Transforms/IPO or unittest/ExecutionEngine/Orc. When building
with BUILD_SHARED_LIBRARIES, such tests don't manage to find their libraries.

Since the tests are run from the build directory, it makes sense to set their
RPATH for the build tree, rather than the install tree. This is the default in
CMake since 2.6, so all we have to do is set CMAKE_BUILD_WITH_INSTALL_RPATH to
OFF for the unit tests.

llvm-svn: 280791