Increase libcudacxx test timeout (#3850)

* Increase libcu++ test timeout.

The T4 nightlies are consistently timing out.

libcudacxx/test/libcudacxx/CMakeLists.txt

@@ -140,6 +140,6 @@ add_test(NAME libcudacxx.test.lit COMMAND
 )
 
 set_tests_properties(libcudacxx.test.lit PROPERTIES
-  TIMEOUT 6400
+  TIMEOUT 10800 # 3hr, some CI machines are slow
   RUN_SERIAL TRUE
 )