gather facts improvements (#44008)

* gather_subset is a list of strings

When gather_subset is an integer, a message pointing out the problem, current tb error 
is replaced by this one:

    ERROR! the field 'gather_subset' should be a list of (<class 'str'>,), but the item '42' is a <class 'int'>
...
* gathering_facts test: ensure smart gathering is on

test/integration/targets/gathering_facts/test_run_once.yml

@@ -2,6 +2,10 @@
 - hosts: facthost1
   gather_facts: no
   tasks:
+    - name: check that smart gathering is enabled
+      fail:
+        msg: 'smart gathering must be enabled'
+      when: 'lookup("env", "ANSIBLE_GATHERING") != "smart"'
     - name: install test local facts
       copy:
         src: uuid.fact
@@ -13,14 +17,14 @@
   run_once: yes
   tasks:
     - block:
-      - name: 'Check the same host is used'
-        assert:
-          that: 'hostvars.facthost1.ansible_fqdn == hostvars.facthost2.ansible_fqdn'
-          msg: 'This test requires 2 inventory hosts referring to the same host.'
-      - name: "Check that run_once doesn't prevent fact gathering (#39453)"
-        assert:
-          that: 'hostvars.facthost1.ansible_local.uuid != hostvars.facthost2.ansible_local.uuid'
-          msg: "{{ 'Same value for ansible_local.uuid on both hosts: ' ~ hostvars.facthost1.ansible_local.uuid }}"
+        - name: 'Check the same host is used'
+          assert:
+            that: 'hostvars.facthost1.ansible_fqdn == hostvars.facthost2.ansible_fqdn'
+            msg: 'This test requires 2 inventory hosts referring to the same host.'
+        - name: "Check that run_once doesn't prevent fact gathering (#39453)"
+          assert:
+            that: 'hostvars.facthost1.ansible_local.uuid != hostvars.facthost2.ansible_local.uuid'
+            msg: "{{ 'Same value for ansible_local.uuid on both hosts: ' ~ hostvars.facthost1.ansible_local.uuid }}"
       always:
         - name: remove test local facts
           file: