[C++] Move implementations in arrow/ipc/test-common.h into libarrow_testing

[asfimport]

Instead of having these all defined as static inline we should move the implementations into a testing support library.

Reporter: Uwe Korn / @xhochy
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Add support for reading Flight streams with dictionaries (is superceded by)

_{Note: This issue was originally created as ARROW-4904. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
This was fixed as part of ARROW-3200.