ci: Run RunCMake.ExternalProject serially in macos-x86_64 Xcode job

Running tests concurrently makes the machine busy enough that the
`INACTIVITY_TIMEOUT` test cases timeout.  Run it serially.

.gitlab/ci/configure_macos_x86_64_xcode.cmake

@@ -1,2 +1,3 @@
+set(CMake_TEST_RunCMake_ExternalProject_RUN_SERIAL ON CACHE BOOL "")
 include("${CMAKE_CURRENT_LIST_DIR}/configure_macos_common.cmake")
 include("${CMAKE_CURRENT_LIST_DIR}/configure_external_test.cmake")