tests: move common definitions and functions into pidfd.h

Move definitions and functions used across different pidfd tests into
pidfd.h header.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Christian Brauner <christian@brauner.io>
Link: https://lore.kernel.org/r/20190726162226.252750-1-surenb@google.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>