ci: support a max parallelism for tests

Machines may have more cores than testing actually supports.

.gitlab/ci/ctest_test.cmake

@@ -10,6 +10,11 @@ ctest_start(APPEND)
 
 include(ProcessorCount)
 ProcessorCount(nproc)
+if (NOT "$ENV{CTEST_MAX_PARALLELISM}" STREQUAL "")
+  if (nproc GREATER "$ENV{CTEST_MAX_PARALLELISM}")
+    set(nproc "$ENV{CTEST_MAX_PARALLELISM}")
+  endif ()
+endif ()
 
 include("${CMAKE_CURRENT_LIST_DIR}/ctest_exclusions.cmake")
 ctest_test(