Added per-test priority support

In each module, a variable test_prio[] may be added which
is indexed by the test name.

Currently the only application of this is to exclude tests
from the "default" set of tests that are run:  If a test
has a negative priority, then it is not run by default.  This
addresses the issue with "fill exact match table" taking almost
an hour to run.
diff --git a/tests/basic.py b/tests/basic.py
index 0e91ad5..465a3c9 100644
--- a/tests/basic.py
+++ b/tests/basic.py
@@ -38,6 +38,8 @@
 #@var basic_config Local copy of global configuration data
 basic_config = None
 
+test_prio = {}
+
 def test_set_init(config):
     """
     Set up function for basic test classes
@@ -101,6 +103,8 @@
             basic_logger.error("** FAILED ASSERTION: " + msg)
         unittest.TestCase.assertTrue(self, cond, msg)
 
+test_prio["SimpleProtocol"] = 1
+
 class SimpleDataPlane(SimpleProtocol):
     """
     Root class that sets up the controller and dataplane