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

crates/extensions_ui/src/extensions_ui.rs

@@ -3,9 +3,6 @@ mod extension_registration_hooks;
 mod extension_suggest;
 mod extension_version_selector;
 
-#[cfg(test)]
-mod extension_store_test;
-
 pub use extension_registration_hooks::ConcreteExtensionRegistrationHooks;
 
 use std::ops::DerefMut;