Fix unit test regressions from upstream changes in CargoTool.
Review Request #12960 — Created April 17, 2023 and submitted
cargoslightly changed their output in more recent versions, changing
from a "main" thread to a per-test thread in the output reporting. This
broke our unit tests, but didn't impact any other behavior.
This fixes up our unit tests to pass with modern versions.
Unit tests pass.