selftests: forwarding: Allow running specific tests

Similar to commit a511858c

 ("selftests: fib_tests: Allow user to run
a specific test"), allow user to run only a subset of the tests using
the TESTS environment variable.

This is useful when not all the tests can pass on a given system.

Example:
# export TESTS="ping_ipv4 ping_ipv6"
# ./bridge_vlan_aware.sh
TEST: ping					[PASS]
TEST: ping6					[PASS]

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