[C++][FlightRPC] Move the FlightTestServer to its own .cc and .h files

[felipecrv]

Describe the enhancement requested

It's currently too hidden in the generically named test_util.{h,cc} files making it hard to find an example implementation of FlightServerBase.

Component(s)

C++, FlightRPC

[felipecrv]

Issue resolved by pull request 43678
#43678