Build extension_flat_tensor when tests are built (#8481)

Add flat tensor to test cmake

Author: lucylq

CMakeLists.txt

@@ -245,7 +245,7 @@ cmake_dependent_option(
 )
 
 if(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR)
-  set(EXECUTORCH_BUILF_EXTENSION_DATA_LOADER ON)
+  set(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER ON)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_TRAINING)
@@ -348,6 +348,7 @@ if(EXECUTORCH_BUILD_PTHREADPOOL)
 endif()
 
 if(EXECUTORCH_BUILD_TESTS)
+  set(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR ON)
   include(CTest)
 endif()