test: env: add some test cases for new "env default -k" flag

test: env: add some test cases for new "env default -k" flag

Check that the new -k flag works as expected.

This also adds a test of the -a flag, which was previously missing,
and as the comment says, perhaps for a good reason. At least now we
have a test for it in combination with -k (and -f, because the ethaddr
variables otherwise cause complaining).

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>