selftests: forwarding: lib: Add in_ns()

selftests: forwarding: lib: Add in_ns()

In order to run a certain command inside another network namespace, it's
possible to use "ip netns exec ns command". However then one can't use
functions defined in lib.sh or a test suite.

One option is to do "ip netns exec ns bash -c command", provided that
all functions that one wishes to use (and their dependencies) are
published using "export -f". That may not be practical.

Therefore, introduce a helper in_ns(), which wraps a given command in a
boilerplate of "ip netns exec" and "source lib.sh", thus making all
library functions available. (Custom functions that a script wishes to
run within a namespace still need to be exported.)

Because quotes in "$@" aren't recognized in heredoc, hand-expand the
array in an explicit for loop, leveraging printf %q to handle proper
quoting.

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