selftests: mlxsw: mirror_gre: Obey TESTS

This test is unusual in that overriding TESTS does not change the tests to
be run. Split the individual tests into several functions and invoke them
through tests_run() as appropriate.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>