Add tests for some extensions currently not covered by the regression tests (#11191)

## Problem
Some extensions do not contain tests, which can be easily run on top of
docker-compose or staging.

## Summary of changes
Added the pg_regress based tests for `pg_tiktoken`, `pgx_ulid`, `pg_rag`
Now they will be run on top of docker-compose, but I intend to adopt
them to be run on top staging in the next PRs

docker-compose/ext-src/pgrag-src/sql/text_processing.sql

@@ -0,0 +1,4 @@
+-- Text processing function tests
+SELECT rag.markdown_from_html('<p>Hello <i>world</i></p>');
+
+SELECT rag.chunks_by_character_count('the cat sat on the mat', 10, 5);