Created by: robbertvanginkel
For usage with the https://github.com/facebook/ios-snapshot-test-case repo.
Snapshot test reference image paths are controlled by an environment variable that is set on the test run process. Currently though it is not possible to add an environment variable to a specific test target. This commit allows indivudual apple_test rules to reference different paths for snapshot test reference images.
This is a specific field instead of a more general thing like test_environment_variables, to allow the value to be a SourcePath so it can be combined with the export_file rule.