ctest_submit: show headers as debug info

Hide any headers specified by `ctest_submit(HTTPHEADER ...)` unless ctest
is run with --debug.  This option is used to present bearer tokens to CDash.
We do not want to inadvertently expose this secret data in the console
output log.
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager