Add testing for extensions (#7818)

## Problem

We need automated tests of extensions shipped with Neon to detect
possible problems.

## Summary of changes

A new image neon-test-extensions is added. Workflow changes to test the
shipped extensions are added as well.
Currently, the regression tests, shipped with extensions are in use.
Some extensions, i.e. rum, timescaledb, rdkit, postgis, pgx_ulid, pgtap,
pg_tiktoken, pg_jsonschema, pg_graphql, kq_imcx, wal2json_2_5 are
excluded due to problems or absence of internal tests.

---------

Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

docker-compose/compute_wrapper/var/db/postgres/specs/spec.json

@@ -95,7 +95,7 @@
             },
             {
                 "name": "shared_preload_libraries",
-                "value": "neon",
+                "value": "neon,pg_cron,timescaledb,pg_stat_statements",
                 "vartype": "string"
             },
             {
@@ -127,6 +127,16 @@
                 "name": "max_replication_flush_lag",
                 "value": "10GB",
                 "vartype": "string"
+            },
+            {
+                "name": "cron.database",
+                "value": "postgres",
+                "vartype": "string"
+            },
+            {
+                "name": "session_preload_libraries",
+                "value": "anon",
+                "vartype": "string"
             }
         ]
     },