refactor(optimized-ssr-engine.spec): don't confuse terms: request, re…

…nder, HTML, response (#19369)

Terms used in tests now comply with the following semantics:
- `request` - is sent by client and received by the OptimizedSsrEngine
- `response` - is sent back to the client by the OptimizedSsrEngine
- `render` / `rendering` - the process performed by Angular to produce a final HTML (or error) for given rendering key (note: the rendering key is by default the URL path in the `request`)
- `HTML` - the result of the rendering

closes https://jira.tools.sap/browse/CXSPA-7860