Improve SSR v2 test coverage

[nolanlawson]

Test coverage for ssr-compiler and ssr-runtime is lower than the rest of the codebase: https://github.com/salesforce/lwc/pull/5139 . Ideally it should be close to 100% since this is brand-new code.

Coverage could likely be improved by 1) adding tests, or 2) moving some duplicated code into @lwc/shared.

[git2gus[bot]]

This issue has been linked to a new work item: W-17625416