Actually, belay that, your session relevance doesn't look correct. The "client" object only exists in Client Relevance, not Session Relevance.
Easiest to debug Session Relevance queries using your Web Reports server, via
I think what you are looking for is
client settings of <bes computer>: bes client setting
q: properties of type "bes client setting"
A: name of <bes client setting>: string
A: scope of <bes client setting>: string
A: value of <bes client setting>: string
names of bes computers whose (not exists client settings whose (name of it = "Test Setting") of it)