selftests: rtnetlink: Make the set of tests to run configurable

Extract the list of all tests into a variable, ALL_TESTS. Then assume the
environment variable TESTS holds the list of tests to actually run, falling
back to ALL_TESTS if TESTS is empty. This is the same interface that
forwarding selftests use to make the set of tests to run configurable.
In addition to this, allow setting the value explicitly through a command
line option "-t" along the lines of what fib_nexthops.sh does.

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