Improve pattern text downgrade license test

[parkertimmins]

Update the pattern_text downgrade test so that it include adding docs and querying for docs.
Specifically the test now does the following:

1. Create data stream with trial license and pattern text field
2. Index docs, verify they're searchable.
3. Downgrade to basic license.
4. Index more docs in same backing index, verify all docs searchable, verify settings unchanged.
5. Rollover the data stream, verify the new backing index has disable_templating=true.
6. Index more docs into the new backing index, verify all docs searchable across both indices.
7. Search with "fields": ["pattern_field"] to verify the valueFetcher() code path works across both backing indices.

[parkertimmins]

Added fields loading query so that it would have caught the bug fixed in #142767 . (This test should fail until that PR is merged)