tests: Fix conntrack tests on windows.

tests: Fix conntrack tests on windows.

The conntrack unit tests seem to generate different megaflow masks on
Windows.  The megaflow masks depend on the internal ordering of the
subtables, which are sorted using qsort(), based on their max priority.
If two subtables have the same priority the ordering between them
depends on the stability properties of qsort(), which apparently are
different between Windows and Linux/*BSD.

This commit uses multiple OpenFlow tables to build our conntrack
pipelines in the tests, which gives us more control over the visited
subtables and also improves clarity.

Reported-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>