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>

.github/workflows/build_and_test.yml

@@ -858,6 +858,26 @@ jobs:
           cache-to: type=registry,ref=neondatabase/compute-node-${{ matrix.version }}:cache-${{ matrix.arch }},mode=max
           tags: |
             neondatabase/compute-node-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}-${{ matrix.arch }}
+      
+      - name: Build neon extensions test image
+        if: matrix.version == 'v16'
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          build-args: |
+            GIT_VERSION=${{ github.event.pull_request.head.sha || github.sha }}
+            PG_VERSION=${{ matrix.version }}
+            BUILD_TAG=${{ needs.tag.outputs.build-tag }}
+            TAG=${{ needs.build-build-tools-image.outputs.image-tag }}
+          provenance: false
+          push: true
+          pull: true
+          file: Dockerfile.compute-node
+          target: neon-pg-ext-test
+          cache-from: type=registry,ref=neondatabase/neon-test-extensions-${{ matrix.version }}:cache-${{ matrix.arch }}
+          cache-to: type=registry,ref=neondatabase/neon-test-extensions-${{ matrix.version }}:cache-${{ matrix.arch }},mode=max
+          tags: |
+            neondatabase/neon-test-extensions-${{ matrix.version }}:${{needs.tag.outputs.build-tag}}-${{ matrix.arch }}
 
       - name: Build compute-tools image
         # compute-tools are Postgres independent, so build it only once
@@ -902,6 +922,13 @@ jobs:
                                              neondatabase/compute-node-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}-x64 \
                                              neondatabase/compute-node-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}-arm64
 
+      - name: Create multi-arch neon-test-extensions image
+        if: matrix.version == 'v16'
+        run: |
+          docker buildx imagetools create -t neondatabase/neon-test-extensions-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }} \
+                                             neondatabase/neon-test-extensions-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}-x64 \
+                                             neondatabase/neon-test-extensions-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}-arm64
+
       - name: Create multi-arch compute-tools image
         if: matrix.version == 'v16'
         run: |
@@ -1020,7 +1047,7 @@ jobs:
             exit 1
           fi
 
-      - name: Verify docker-compose example
+      - name: Verify docker-compose example and test extensions
         timeout-minutes: 20
         run: env TAG=${{needs.tag.outputs.build-tag}} ./docker-compose/docker_compose_test.sh