net/mlx5e: loopback test is not supported in switchdev mode

In switchdev mode we insert steering rules to eswitch that
make sure packets can't be looped back.
Modify the self tests infra and have flags per test.
Add a flag for tests that needs to be skipped in switchdev mode.

Before this commit:

$ ethtool --test enp8s0f0
 The test result is FAIL
 The test extra info:
 Link Test        0
 Speed Test       0
 Health Test      0
 Loopback Test    1

After this commit:

$ ethtool --test enp8s0f0
 The test result is PASS
 The test extra info:
 Link Test        0
 Speed Test       0
 Health Test      0

Example output in dmesg:

enp8s0f0: Self test begin..
enp8s0f0:         [0] Link Test start..
enp8s0f0:         [0] Link Test end: result(0)
enp8s0f0:         [1] Speed Test start..
enp8s0f0:         [1] Speed Test end: result(0)
enp8s0f0:         [2] Health Test start..
enp8s0f0:         [2] Health Test end: result(0)
enp8s0f0: Self test out: status flags(0x1)

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>