Move ExtensionStore tests back to extension_host (#20682)

This PR moves the tests for the `ExtensionStore` back into the
`extension_host` crate.

We now have a separate `TestExtensionRegistrationHooks` to use in the
test that implements the minimal required functionality needed for the
tests. This means that we can depend on the `theme` crate only in the
tests.

Release Notes:

- N/A

Cargo.lock

@@ -4189,6 +4189,7 @@ dependencies = [
  "serde_json_lenient",
  "settings",
  "task",
+ "theme",
  "toml 0.8.19",
  "url",
  "util",
@@ -4203,36 +4204,26 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "assistant_slash_command",
- "async-compression",
- "async-tar",
  "client",
  "collections",
  "context_servers",
- "ctor",
  "db",
  "editor",
- "env_logger 0.11.5",
  "extension",
  "extension_host",
  "fs",
- "futures 0.3.30",
  "fuzzy",
  "gpui",
- "http_client",
  "indexed_docs",
  "language",
  "log",
  "lsp",
- "node_runtime",
  "num-format",
- "parking_lot",
  "picker",
  "project",
  "release_channel",
- "reqwest_client",
  "semantic_version",
  "serde",
- "serde_json",
  "settings",
  "smallvec",
  "snippet_provider",