[mlir] [test] Fix unittests in standalone builds (#120910)

Fix the logic used to run unit tests to account for `llvm_gtest` targets
being installed, since 91b3ca3966. This
involves removing a rule that would cause a duplicate `llvm_gtest`
target being created, and updates the method for determining whether
unittests can be run to checking whether the target is present, rather
than the source directory (that is no longer actually necessary).

mlir/CMakeLists.txt

@@ -27,11 +27,6 @@ if(MLIR_STANDALONE_BUILD)
 
   include_directories(${LLVM_INCLUDE_DIRS})
 
-  set(UNITTEST_DIR ${LLVM_THIRD_PARTY_DIR}/unittest)
-  if(EXISTS ${UNITTEST_DIR}/googletest/include/gtest/gtest.h)
-    add_subdirectory(${UNITTEST_DIR} third-party/unittest)
-  endif()
-
   set(CMAKE_LIBRARY_OUTPUT_DIRECTORY
     "${CMAKE_CURRENT_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}")
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin")
@@ -227,7 +222,7 @@ if (MLIR_INCLUDE_TESTS)
   add_definitions(-DMLIR_INCLUDE_TESTS)
   add_custom_target(MLIRUnitTests)
   set_target_properties(MLIRUnitTests PROPERTIES FOLDER "MLIR/Tests")
-  if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest/googletest/include/gtest/gtest.h)
+  if (TARGET llvm_gtest)
     add_subdirectory(unittests)
   else()
     message(WARNING "gtest not found, unittests will not be available")