ci: pay attention to the machine load when running tests

CMake's test suite is sensitive to machine load and can cause spurious
timeouts. To acknowledge this, prevent CTest from spawning tests into a
busy machine.

.gitlab/ci/ctest_test.cmake

@@ -19,6 +19,7 @@ endif ()
 include("${CMAKE_CURRENT_LIST_DIR}/ctest_exclusions.cmake")
 ctest_test(
   PARALLEL_LEVEL "${nproc}"
+  TEST_LOAD "${nproc}"
   RETURN_VALUE test_result
   EXCLUDE "${test_exclusions}")
 ctest_submit(PARTS Test)

.gitlab/ci/ctest_test_external.cmake

@@ -71,6 +71,7 @@ endif ()
 include("${CMAKE_CURRENT_LIST_DIR}/ctest_exclusions.cmake")
 ctest_test(
   PARALLEL_LEVEL "${nproc}"
+  TEST_LOAD "${nproc}"
   RETURN_VALUE test_result
   ${ctest_label_args}
   EXCLUDE "${test_exclusions}")