bpf/tests: Add test case flag for verifier zero-extension

This patch adds a new flag to indicate that the verified did insert
zero-extensions, even though the verifier is not being run for any
of the tests.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-9-johan.almbladh@anyfinetworks.com