Add framework for testing extensions in TCL

There is a distinction between tests that verify extension-specific
behavior and tests that verify interactions between the database engine
and extensions. Previously, both types of tests were kept in extensions.py.
With this new framework, we can extract the latter type of tests from
extensions.py into TCL. This cleans up extensions.py and provides
compatibility testing with SQLite at no extra cost.

To demonstrate the framework’s usage, tests verifying the handling of
virtual tables were extracted to TCL.

In the future, we may consider moving extension-specific tests to TCL as
well, especially those that have counterparts in SQLite or sqlean.

.github/workflows/rust.yml

@@ -86,5 +86,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: "./.github/shared/install_sqlite"
+      - name: build SQLite test extensions
+        run: cargo build --package limbo_sqlite_test_ext
       - name: Test
         run: SQLITE_EXEC="sqlite3" make test-compat