arrow

GH-43677: [C++][FlightRPC] Move the FlightTestServer to its own .cc and .h files (#43678)

### 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: #43677

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>