Unit test operators used by celestia-core

[rootulp]

[Non blocking][Question]
Does it make sense to add tests also for the other operators supported by core?

https://github.com/celestiaorg/celestia-core/blob/3e636bb02f48cc02b11f4aea3dd1494ef8e62c91/libs/pubsub/query/query.go#L65-L84

Even if they're caught by the regex, so that upcoming changes don't end up breaking the API again

Originally posted by @sweexordious in #331 (comment)