contrib/rpm: untangle setting of bcond_{test,debug} defaults

Resolve the defaults in build.sh instead of RPM macros. This looks less terrible maintaining the same defaults as well as options to override it upstream.

Moving it to the block that downstreams (Fedora, Red Hat) keep customized makes it possible for them to also maintain customized defaults here.

In particular, the downstreams should be able to enable bcond_test at least for their production release (otherwise there's little point in actually running tests at package build time).