varnames: add tests, fix exception grammar (#70573)

Change:
- Add integration tests for various cases
- Fix wrong use of "its" in an exception thrown in varnames when it
  throws an AnsibleError, given a term of the wrong type.

Test Plan:
- new tests, CI

Tickets:
- Fixes #70546

Signed-off-by: Rick Elrod <rick@elrod.me>

changelogs/fragments/varnames-error-grammar.yml

@@ -0,0 +1,2 @@
+minor_changes:
+  - varnames lookup plugin - Fixed grammar error in exception message when the plugin is given a non-string term.

lib/ansible/plugins/lookup/varnames.py

@@ -66,7 +66,7 @@ class LookupModule(LookupBase):
         for term in terms:
 
             if not isinstance(term, string_types):
-                raise AnsibleError('Invalid setting identifier, "%s" is not a string, its a %s' % (term, type(term)))
+                raise AnsibleError('Invalid setting identifier, "%s" is not a string, it is a %s' % (term, type(term)))
 
             try:
                 name = re.compile(term)

test/integration/targets/lookup_varnames/aliases

@@ -0,0 +1,2 @@
+shippable/posix/group2
+skip/python2.6  # lookups are controller only, and we no longer support Python 2.6 on the controller

test/integration/targets/lookup_varnames/tasks/main.yml

@@ -0,0 +1,38 @@
+# Example copied from docs
+- name: Set some variables
+  set_fact:
+    qz_1: hello
+    qz_2: world
+    qa_1: "I won't show"
+    qz_: "I won't show either"
+
+- name: Try various regexes and make sure they work
+  assert:
+    that:
+      - lookup('varnames', '^qz_.+', wantlist=True) == ['qz_1', 'qz_2']
+      - lookup('varnames', '^qz_.+', '^qa.*', wantlist=True) == ['qz_1', 'qz_2', 'qa_1']
+      - "'ansible_python_interpreter' in lookup('varnames', '^ansible_.*', wantlist=True)"
+      - lookup('varnames', '^doesnotexist.*', wantlist=True) == []
+      - lookup('varnames', '^doesnotexist.*', '.*python_inter.*', wantlist=True) == ['ansible_python_interpreter']
+      - lookup('varnames', '^q.*_\d', wantlist=True) == ['qz_1', 'qz_2', 'qa_1']
+      - lookup('varnames', '^q.*_\d') == 'qz_1,qz_2,qa_1'
+
+- name: Make sure it fails successfully
+  set_fact:
+    fail1: "{{ lookup('varnames', True, wantlist=True) }}"
+  register: fail1res
+  ignore_errors: yes
+
+- name: Make sure it fails successfully
+  set_fact:
+    fail2: "{{ lookup('varnames', '*', wantlist=True) }}"
+  register: fail2res
+  ignore_errors: yes
+
+- assert:
+    that:
+      - fail1res is failed
+      - "'Invalid setting identifier' in fail1res.msg"
+      - fail2res is failed
+      - "'Unable to use' in fail2res.msg"
+      - "'nothing to repeat' in fail2res.msg"