Write some tests for ethtool WoL configuration

[lunn]

Many MAC and PHY drivers get WoL wrong. It would be good to have some
tests of the ethtool API exercising this API, for real MAC/PHY
drivers, showing that the system can be woken.

It probably makes sense to consider two hardware setups:

1. A single interface connected to a link peer which the test cannot
   control.

2. Two hosts, two interfaces, linked by a cable.

With 1) some basic local API tests can be made, but actual
suspend/wake cannot be tested. With 2) all WoL options should be
testable. However, some thought must be put into the problem of failed
tests, the host remains powered off...

Work items:

Write a wall of text for a test specification. Trawl the netdev list
to find the common errors, and ensure the tests will detect them. The
aim should be this test can be become comments in the test
implementation aiding developers to understand why their
implementation fails tests.

Review by subject experts, fixup text, rinse, repeat

Implement the tests, and test on a couple of different hardware
platforms, ideally some that are broken.