Add missing calls to `act` in `useStoreProxy` tests

Add calls to `act` around proxy method calls that trigger store changes
which in turn schedule deferred updates within Preact components. This
should hopefully fix an issue with flakey tests due to updates being
flushed after the tests have finished executing.

src/sidebar/store/test/use-store-test.js

@@ -82,7 +82,10 @@ describe('sidebar/store/use-store', () => {
       assert.deepEqual(proxy.getThing('bar'), { id: 'bar' });
 
       // Test proxied action dispatch.
-      proxy.addThing('baz');
+      act(() => {
+        proxy.addThing('baz');
+      });
+
       assert.calledWith(addThingSpy, 'baz');
     });
 
@@ -118,8 +121,10 @@ describe('sidebar/store/use-store', () => {
 
       const { proxy } = renderTestComponent();
 
-      proxy.addThing('foo');
-      proxy.addThing('foo');
+      act(() => {
+        proxy.addThing('foo');
+        proxy.addThing('foo');
+      });
 
       assert.calledTwice(addThingSpy);
       assert.calledWith(addThingSpy, 'foo');