[fix] prefer context from constructor options

[Conduitry]

This fixes #6753 and also fixes it for SSR so that the passed context is preferred, even when we have a parent_component. Browser and SSR are implemented as separate tests, because the process of synchronously creating a child component outside of Svelte's jurisdiction is rather different in the two cases.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint