ethtool: use serial-tests config needed by ptest.

buildtest-TESTS and runtest-TESTS targets are required by ptest.
In order to have those targets in automake 1.13.4 serial-tests
should be specified since parallel-tests is assumed by default
and serial-tests is optional.

ptest results:
PASS: test-cmdline
PASS: test-features
==================
All 2 tests passed
==================

Signed-off-by: Tudor Florea <tudor.florea@enea.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>