Test fails on Debian experimental reproducible builds

A recent build on the reproducible build machines with Debian experimental fails on one test.

util/num_cpus: 
  FAIL ../src/test/test_util.c:3689: assert(num OP_LE 16): 18 vs 16
  [num_cpus FAILED]

This test was added in commit 603cb712ef756dd700a52e837bcd643a96311ad6 which expects the maximum number of CPUs to be 16. The compute_num_cpus function only logs a message for machines with more than 16 CPUs but doesn't clamp the return value to 16. So there is a discrepancy between the implementation and the test. (Why is there a limit anyway?)

Furthermore, the preprocessor macro that defines the maximum number of CPUs isn't public and can't be used in tests leading to undefined magic numbers.