GH-43677: [C++][FlightRPC] Move the FlightTestServer to its own .cc and .h files

[felipecrv]

Rationale for this change

One way of learning about a codebase is reading the tests. As it is now, it's hard to see the minimal FlightServerBase sub-class in flight/test_util.cc, so I moved it to its own file.

What changes are included in this PR?

• Renaming FlightTestServer to TestFlightServer
• Moving the class to test_flight_server.{h,cc}
• Bonus: Moving the server and client auth handlers to test_auth_handlers.{h,cc}

Are these changes tested?

By existing tests.

Are there any user-facing changes?

ExampleTestServer is removed from the testing library in favor of FlightTestServer::Make.

• GitHub Issue: [C++][FlightRPC] Move the FlightTestServer to its own .cc and .h files #43677

[felipecrv]

Pushed a commit moving the auth handlers as well.

[lidavidm]

I'll just note that these are very much not meant as pedagogical examples

[kou]

+1

[kou]

Do we need this factory method?
How about using std::make_unique<TestFlightServer>() directly in callers?

[felipecrv]

It's used quite a lot and is handy to not have to write the longer make_unique invocation. This is replacing the old free function called ExampleTestServer.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 69bce8f.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 4 possible false positives for unstable benchmarks that are known to sometimes produce them.