Add testing using Ansible Molecule

Test the role on the supported platforms specified in commit f1dc9d3.

.github/workflows/molecule-requirements.txt

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+ansible>=9.1.0,<10.0.0
+ansible-core~=2.16
+molecule>=6.0.2,<7.0.0
+molecule-plugins[podman]>=23.5.0,<24.0.0

.github/workflows/molecule.yaml

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+---
+name: Ansible Molecule
+
+"on":
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  molecule:
+    name: Molecule
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Check out the codebase.
+        uses: actions/checkout@v4
+
+      - name: Setup Python.
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.12"
+          cache: "pip"
+          cache-dependency-path: .github/workflows/molecule-requirements.txt
+
+      - name: Install Ansible Molecule.
+        run: pip install --user -r .github/workflows/molecule-requirements.txt
+
+      - name: Run Ansible Molecule.
+        run: |
+          molecule --base-config molecule/base.yml test
+        env:
+          ANSIBLE_FORCE_COLOR: '1'

molecule/README.md

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+# Ansible Molecule tests
+
+Use the base configuration file when calling molecule, like this.
+
+```shell
+molecule --base-config molecule/base.yml test
+```

molecule/base.yml

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+---
+dependency:
+  name: galaxy
+driver:
+  name: podman
+platforms:
+  - name: el-7
+    image: centos:7
+  - name: el-8
+    image: rockylinux:8
+  - name: el-9
+    image: rockylinux:9
+  - name: ubuntu-focal
+    image: ubuntu:focal
+  - name: ubuntu-jammy
+    image: ubuntu:jammy
+  - name: ubuntu-lunar
+    image: ubuntu:lunar
+  - name: ubuntu-mantic
+    image: ubuntu:mantic
+provisioner:
+  name: ansible
+  config_options:
+    defaults:
+      interpreter_python: auto_silent
+      callback_whitelist: profile_tasks, timer, yaml
+    ssh_connection:
+      pipelining: true
+  inventory:
+    host_vars:
+      el-7:
+        ansible_python_interpreter: /usr/bin/python
+      el-8:
+        ansible_python_interpreter: /usr/bin/python3
+verifier:
+  name: ansible

molecule/default/converge.yml

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+---
+- name: Converge
+  hosts: all
+  vars_files:
+    - vars.yml
+  tasks:
+    - name: "Include galaxyproject.gxadmin"
+      ansible.builtin.include_role:
+        name: "galaxyproject.gxadmin"

molecule/default/vars.yml

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+---