Update test to retry assertion on empty file and don't include forward-slash in the assertion

[philipp-spiess]

To fix the CI issues we have, it turns out there are two issues:

1. The file-read was not retried, making it possible for all platforms (but mostly Windows because it's the slowest) to sometimes not have the file created yet before making the assertion.
2. A forward-slash in the assertion message path would always be a backslash when run on Windows, thus the Windows test would never pass.

[ci-all]

Test plan

• [ci-all] and have the test pass two times.