Generate /etc/hosts with Ansible

main.yml

# Idempotent way to build a /etc/hosts file with Ansible using your Ansible hosts inventory for a source.
# Will include all hosts the playbook is run on.
# Inspired from http://xmeblog.blogspot.com/2013/06/ansible-dynamicaly-update-etchosts.html

- name: "Build hosts file"
  lineinfile: dest=/etc/hosts regexp='.*{{ item }}$' line="{{ hostvars[item].ansible_default_ipv4.address }} {{item}}" state=present
  when: hostvars[item].ansible_default_ipv4.address is defined
  with_items: groups['all']

@enr0s, you can also use the tag/variable molecule-notest that's already set by molecule, so you don't have to define your own.

tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: /etc/hosts
    owner: root
    group: root
    mode: 0644
  tags:
    - molecule-notest

or
tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: /etc/hosts
    owner: root
    group: root
    mode: 0644
  when: molecule-notest not in ansible_skip_tags

Another solution to this issue would be to use a "dummy" hosts file with molecule, which is not a perfect solution but is better than skipping the task in my opinion because you can then confirm that the hosts file is generated correctly.
The way to achieve that would be something like:

defaults/main.yml

---
etc_hosts_file: /etc/hosts

molecule.yml

provisioner:
  name: ansible
  inventory:
    group_vars:
      all:
        etc_hosts_file: /tmp/molecule_etc_hosts

tasks/main.yml

---
- name: Generate /etc/hosts file
  template:
    src: etc/hosts.j2
    dest: "{{ etc_hosts_file }}"
    owner: root
    group: root
    mode: 0644

@dywanik , I am not sure if that is what you are searching for... in my case, I wanted simply to add an entry to /etc/hosts.
The following worked for me:

molecule.yml:

---
...
platforms:
  - name: "myplatformname"
    etc_hosts:
      "repo": "8.8.8.8"