Make `git clean -nxd` silent after `make distclean`

Also check that invariant in CI: After a `make distclean`
`git clean -nxd` should find no files.

The hope is that this catches cases were a `make clean` would forget to
list a `.gitignore`d file that should either be registered as a
`(DIST_)CLEAN_FILES` or should be unignored and committed into the
repository.
1 job for master in 5 minutes and 19 seconds (queued for 16 minutes and 15 seconds)
Status Job ID Name Coverage
  Test
passed #12115
validate

00:05:19