tests/kolainst: Fix failures on container-native FCOS

tests/kolainst: Fix failures on container-native FCOS

On container-native FCOS (F44+), /etc/ostree/remotes.d/ may have no
.conf files since the system uses container-based updates rather than
traditional ostree remotes. This caused test failures:

- staged-deploy.sh and finalization.sh: sed on the unexpanded glob
  '/etc/ostree/remotes.d/*.conf' fails under set -euo pipefail when
  no files match. Fix by iterating with a for loop and guarding with
  test -f.

- itest-remotes.sh: 'ostree remote list' returns empty, hitting
  assert_not_reached. Fix by skipping the test gracefully on systems
  with no remotes configured.

Assisted-by: OpenCode (Claude Opus 4.6)
Signed-off-by: Joseph Marrero Corchado <jmarrero@redhat.com>