Compute the test file name in _finalize_params

Bug: 330885075
Change-Id: Ifaad7186f02ad856c521a41ae2bfbcd0204af21f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5378526


Reviewed-by: Andres Ricardo Perez <andresrperez@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282119}