Use a dedicated test extension in extension tests (#13781)

This PR updates the `extension` crate's tests to use a dedicated test
extension for its tests instead of the real Gleam extension.

As the Gleam extension continues to evolve, it makes it less suitable to
use as a test fixture:

1. For a while now, the test has failed locally due to me having `gleam`
on my $PATH, which causes the extension's `get_language_server_command`
to go down a separate codepath.
2. With the addition of the `indexed_docs_providers` the test was
hanging indefinitely.

While these problems are likely solvable, it seems reasonable to have a
dedicated extension to use as a test fixture. That way we can do
whatever we need to exercise our test criteria.

The `test-extension` is a fork of the Gleam extension with some
additional functionality removed.

Release Notes:

- N/A

extensions/gleam/extension.toml

@@ -24,4 +24,4 @@ description = "Returns Gleam docs."
 requires_argument = true
 tooltip_text = "Insert Gleam docs"
 
-# [indexed_docs_providers.gleam-hexdocs]
+[indexed_docs_providers.gleam-hexdocs]