Tests: Use -A option to pass generator platform selection

This is less verbose than defining CMAKE_GENERATOR_PLATFORM.

Tests/ExternalProjectUpdate/ExternalProjectUpdateTest.cmake

@@ -13,7 +13,7 @@ macro(check_a_tag desired_tag resulting_sha fetch_expected)
   # Configure
   execute_process(COMMAND ${CMAKE_COMMAND}
     -G ${CMAKE_GENERATOR} -T "${CMAKE_GENERATOR_TOOLSET}"
-    -DCMAKE_GENERATOR_PLATFORM=${CMAKE_GENERATOR_PLATFORM}
+    -A "${CMAKE_GENERATOR_PLATFORM}"
     -DTEST_GIT_TAG:STRING=${desired_tag}
     ${ExternalProjectUpdate_SOURCE_DIR}
     WORKING_DIRECTORY ${ExternalProjectUpdate_BINARY_DIR}