test(ssr-compiler): harmonize fixture format

[nolanlawson]

Details

This does two things:

1. Ensure that error.txt and expected.html files are always written, even as empty files. This ensures that when you run yarn test on ssr-compiler, it will not unexpectedly create some additional files, e.g. if it expects no error but gets an error. This creates some additional noise on the filesystem, but makes it much easier to test ssr-compiler and engine-server in parallel.
2. Add an env var, TEST_SSR_COMPILER, that allows for testing the ssr-compiler without having to change the Jest config file.

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.