[SG-21368] Split HoverOverlay styles into multiple files and colocate them with subcomponents

Created by: gitstart-sourcegraph

Description

1. Seperate HoverOverlay css
2. Global CSS classes used in integration tests are replaced with data-testid attributes for .hover-overlay

Refs

• https://github.com/sourcegraph/sourcegraph/issues/21368
• https://app.gitstart.com/clients/sourcegraph/tickets/SG-21368