[offload] Fix enabling unittests in standalone builds (#138418)

Modify the unittest logic in offload to only look for
`third-party/unittest` directory when `llvm_gtest` is not provided by
LLVM itself (in-tree or installed). This makes it possible to run
unittests in sparse checkouts without the `third-party/unittest` tree.

While at it, also make sure `LLVM_THIRD_PARTY_DIR` is actually set while
performing standalone builds. The logic is copied from `compiler-rt`.

---------

Co-authored-by: Joseph Huber <[email protected]>

Author: mgorny

offload/CMakeLists.txt

@@ -386,9 +386,12 @@ add_subdirectory(liboffload)
 add_subdirectory(test)
 
 # Add unit tests if GMock/GTest is present
-if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest)
-  if (NOT TARGET llvm_gtest)
-    add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
-  endif()
+if(NOT LLVM_THIRD_PARTY_DIR)
+  set(LLVM_THIRD_PARTY_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../third-party")
+endif()
+if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
+  add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
+endif()
+if(TARGET llvm_gtest)
   add_subdirectory(unittests)
 endif()