Ben Gamari authored Nov 07, 2019 and Marge Bot committed Nov 09, 2019

As described in #17449, PartialDownsweep is currently fragile due to its
dependence on the error messages produced by the C preprocessor. To eliminate
this dependence we simply ignore stderr output, instead relying on the fact
that the test will exit with a non-zero exit code on failure.

Fixes #17449.