selftests: drivers: hw: ethtool.sh: Adjust output

Some log_test calls are done in a loop, and lead to the same log output.
This might prove tricky to deduplicate for automated tools. Instead, roll
the unique information from log_info to log_test, and drop the log_info.
This also leads to more compact and clearer output.

This change prompts rewording the messages so that they are not excessively
long.

Some check_err messages do not indicate what the issue actually is, so
reword them to say it's a "ping with", like is the case in some other
instances in this test.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>