avocado/core/runner.py: properly define the execution order

Allowing 'None' to be used as a possible value in the list of valid
execution order modes is a way to deal with a default (implicit)
mode.

Let's make this default mode explicit and documented.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/runner.py

@@ -274,6 +274,10 @@ class TestRunner(object):
     """
     DEFAULT_TIMEOUT = 86400
 
+    #: Mode in which this runner should iterate through tests and variants.
+    #: The allowed values are "variants-per-test" or "tests-per-variant"
+    DEFAULT_EXECUTION_ORDER = "variants-per-test"
+
     def __init__(self, job, result):
         """
         Creates an instance of TestRunner class.
@@ -545,7 +549,7 @@ class TestRunner(object):
         :param execution_order: way of iterating through tests/variants
         :return: generator yielding tuple(test_factory, variant)
         """
-        if execution_order in ("variants-per-test", None):
+        if execution_order == "variants-per-test":
             return (self._template_to_factory(template, variant)
                     for template in test_suite
                     for variant in variants.itertests())
@@ -568,7 +572,8 @@ class TestRunner(object):
         :param replay_map: optional list to override test class based on test
                            index.
         :param execution_order: Mode in which we should iterate through tests
-                            resp. variants.
+                                and variants.  If not provided, will default to
+                                :attr:`DEFAULT_EXECUTION_ORDER`.
         :return: a set with types of test failures.
         """
         summary = set()
@@ -595,6 +600,8 @@ class TestRunner(object):
             for test_template in test_suite:
                 test_template[1]["base_logdir"] = self.job.logdir
                 test_template[1]["job"] = self.job
+            if execution_order is None:
+                execution_order = self.DEFAULT_EXECUTION_ORDER
             for test_factory, variant in self._iter_suite(test_suite, variants,
                                                           execution_order):
                 index += 1