test: add infra for mocking passthru ioctls

test: add infra for mocking passthru ioctls

Functions issuing admin/IO passthru ioctls sorely lack unit tests.
It would be great for unit tests not to need a real NVMe controller.
It's also useful to be able to test responses to commands
that might be impossible to trigger with real controllers.

To that end, implement infrastructure for mocking ioctl(),
allowing tests to set expectations for the NVMe passthru ioctls
that will be issued and control the corresponding responses.

The mock library can be used with LD_PRELOAD so that libnvme's ioctl()
calls are redirected from libc. No changes in libnvme itself are needed.

Signed-off-by: Caleb Sander <csander@purestorage.com>