gdb testsuite: Introduce allow_multi_inferior_tests and use it throughout

The Windows port does not support multi-process debugging.  Testcases
that want to exercise multi-process currently FAIL and some hit
cascading timeouts.  Add a new allow_multi_inferior_tests procedure,
meant to be used with require, and sprinkle it throughout testcases as
needed.

Approved-by: Kevin Buettner <kevinb@redhat.com>
Change-Id: I4a10d8f04f9fa10f4b751f140ad0a6d31fbd9dfb