analyzer_testing: Add contextFor2 which takes a File

Work towards https://github.com/dart-lang/sdk/issues/61785

This is a more type-safe API.

Also rename some `path` variables as we often prefix the package:path
import with the name 'path'. `filePath` is also more meaningful.

Change-Id: Ic3234583654944dba3b810ce1493bb143d27002f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/544901
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>