Fix gtest_discover_tests timeout argument (#5149)

`gtest_discover_tests` runs the built unittest executable to create a
distinct CMake target for every individual unittest in each executable.
However, this was previously noted to time out on MacOS frequently
(because MacOS scans newly built executables for viruses, or
something...) but the timeout argument was incorrectly specified.

cmake/AddTritonUnitTest.cmake

@@ -35,5 +35,5 @@ function(add_triton_ut)
   # Without the TEST_DISCOVERY_TIMEOUT, the tests randomly time out on my mac
   # laptop.  I think the issue may be that the very first time you run a program
   # it's a bit slow.
-  gtest_discover_tests(${__NAME} PROPERTIES TEST_DISCOVERY_TIMEOUT 60)
+  gtest_discover_tests(${__NAME} DISCOVERY_TIMEOUT 60)
 endfunction()