Use Autotest for the Open vSwitch test suite.

Use Autotest for the Open vSwitch test suite.

Autotest doesn't provide a lot of benefit by itself but it does allow us
to easily put a wrapper around each test by using a macro to invoke it.
(To do that with the built-in Automake test framework you need to write
or generate a separate wrapper script for each test, which is a pain.)
The next commit will take advantage of this possibility by adding support
for code coverage analysis.