contrib/rpm: enable -Werror on --with debug instead of --with test

The test bcond is used to make test suite failure terminate the %check
phase. That should generally be done for distro builds.

What the distro builds shouldn't do is terminate the build on compiler
warnings, because they're fairly likely to appear on toolchain updates
without indicating actual bugs.

However, currently that's precisely what do we do now.

Let's use -Werror on debug builds instead. They are intentionally made
more prone to fail (e.g. trip more runtime assertions) because failures
can be more rapidly addressed and don't affect distro builds.
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