[offload] Fix enabling unittests in standalone builds

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`.

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()