vsock/test: Add macros to identify transports

vsock/test: Add macros to identify transports

Add three new macros: TRANSPORTS_G2H, TRANSPORTS_H2G and
TRANSPORTS_LOCAL.
They can be used to identify the type of the transport(s) loaded when
using the `get_transports()` function.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250630-test_vsock-v5-1-2492e141e80b@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>