Add tests to validate vars_files first found behavior (#79386)

test/integration/targets/vars_files/aliases

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+shippable/posix/group3
+context/controller

test/integration/targets/vars_files/inventory

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+[testgroup]
+testhost foo=bar
+testhost2 foo=baz

test/integration/targets/vars_files/runme.sh

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+#!/usr/bin/env bash
+
+set -eux
+
+ansible-playbook runme.yml -i inventory -v "$@"

test/integration/targets/vars_files/runme.yml

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+---
+- hosts: testgroup
+  gather_facts: no
+  vars_files:
+    - "vars/common.yml"
+    -
+      - "vars/{{ foo }}.yml"
+      - "vars/defaults.yml"
+  tasks:
+    - import_tasks: validate.yml
+
+- hosts: testgroup
+  gather_facts: no
+  vars:
+    _vars_files:
+        - 'vars/{{ foo }}.yml'
+        - 'vars/defaults.yml'
+  vars_files:
+    - "vars/common.yml"
+    - "{{ lookup('first_found', _vars_files) }}"
+  tasks:
+    - import_tasks: validate.yml

test/integration/targets/vars_files/validate.yml

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+- assert:
+    that:
+      - common is true
+- assert:
+    that:
+      - is_bar is true
+  when: inventory_hostname == 'testhost'
+- assert:
+    that:
+      - is_bar is false
+  when: inventory_hostname == 'testhost2'

test/integration/targets/vars_files/vars/bar.yml

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+is_bar: yes

test/integration/targets/vars_files/vars/common.yml

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+common: yes

test/integration/targets/vars_files/vars/defaults.yml

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+is_bar: no