Ensure CSS is applied to nested route layouts

[rodoch]

Fixes #1579.

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
• 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

[benmccann]

Thanks for this! Would it be possible to put the test in the existing css test directory? I.e. copy the src/routes/nested/ directory into that project. Each new project we adds has to be compiled and start/stop a new server, so it will keep the tests faster if we minimize the number of test projects. If there's some conflict with the existing one then we wouldn't necessarily need to prioritize that though

[rodoch]

No problem, that's done!