feat: tighten file reading in tests.
feat: tighten file reading in tests.

It turns out our compiler host would occasionally just read files off the file
system, e.g. if an expected path ended up absolute instead of relative.

This change relativizes paths and always throws for unexpected files.

angular/tsickle,ncfxy/tsickle,drjanitor/tsickle,alexeagle/tsickle,alexeagle/tsickle,lucidsoftware/tsickle,drjanitor/tsickle,angular/tsickle,drjanitor/tsickle,ncfxy/tsickle,lucidsoftware/tsickle,lucidsoftware/tsickle,hess-g/tsickle,ncfxy/tsickle,hess-g/tsickle,drjanitor/tsickle,alexeagle/tsickle