selftests/net/forwarding: teamd command not found

selftests/net/forwarding: teamd command not found

Running "make kselftest TARGETS=net/forwarding" results in
multiple ccurrences of the same error:
- ./lib.sh: line 787: teamd: command not found

This patch adds the variable $REQUIRE_TEAMD in every test that uses the
command teamd and checks the $REQUIRE_TEAMD variable in the file "lib.sh"
to skip the test if the command is not installed.

Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Link: https://patch.msgid.link/20250114003323.97207-1-alessandro.zanni87@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>