selftests: forwarding: mirror_gre_lib: Extract generic functions

For non-GRE mirroring tests, a functions along the lines of
do_test_span_gre_dir_ips() and test_span_gre_dir_ips() are necessary,
but such that they don't assume tunnels are involved. Extract the code
from mirror_gre_lib.sh to mirror_lib.sh and convert to just use a given
device without assuming it's named "h3-$tundev". Convert the two
above-mentioned functions to wrappers that pass along the correct device
name.

Add test_span_dir() and fail_test_span_dir() to round up the API for use
by following patches.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>