The base class facilitates generating a server side response test instance,
that gets serialized as xcontent, which then gets parsed into a hlrc response
instance, which then gets asserted against the server side response instance.
This way of testing is more realistic then how hlrc response classes are tested
today, which basically tests that serialization works by generating
hlrc response instance, serialize that to xcontent and then parse it back
to a hlrc response instance.
Besides adding a base test class, this change also cuts AcknowledgedResponseTests
and BroadcastResponseTests over to use this base class.
Relates to #39745
a478196f79