web: faster SearchResultsList test

Created by: sqs

Previously this test took ~33s to run. Now it takes ~3s (which is about as good as it'll get because it uses the DOM). The reason it took so long was that the resultCount values were needlessly high, and the algorithm appears to be superlinear. The component itself does not appear to have the same performance problem; it only occurs using the test mocks (eg MockVisibilitySensor), probably because after-scroll callbacks are debounced in the browser automatically.