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).
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