Fix setup of offline tests

Relates to: gitlab-org/gitlab#390287 (comment 1479361220)

Makes sure that the IP Tables are setup correctly using the same setup as used in the python-pipenv test project.

Additionally we add FF_ENABLE_BASH_EXIT_CODE_CHECK to make sure that if a command fails the script fails. It's already set in https://gitlab.com/gitlab-org/security-products/ci-templates/-/blob/master/includes-dev/qa-dependency_scanning.yml

Testing

• Default image: job (this is taken from this MR pipeline)
  • It installs iptables using apt : logs
  • It says it's offline. logs
  • The analyzer succeeds. logs
• FIPS image: job
  • It installs iptables using dnf. logs
  • It says it's offline. logs
  • The analyzer succeeds. logs