testing/testcase: Simplify setUpClass override

Some advanced tests setting up clusters need to being able to come in half of
the setup process to do some actions on the cluster in order to have its setup
finished, thus allow to override extracted _setUpClass and separated
waitForInstance, so that those actions can be executed.

slapos/testing/testcase.py

@@ -368,14 +368,23 @@ class SlapOSInstanceTestCase(unittest.TestCase):
 
   # Unittest methods
   @classmethod
-  def setUpClass(cls):
-    """Request an instance.
-    """
-    cls._instance_parameter_dict = cls.getInstanceParameterDict()
-    snapshot_name = "{}.{}.setUpClass".format(cls.__module__, cls.__name__)
-
+  def waitForInstance(cls):
+    # waitForInstance does not tolerate any error but with instances,
+    # promises sometimes fail on first run, because services did not
+    # have time to start.
+    # To make debug usable, we tolerate instance_max_retry-1 errors and
+    # only debug the last.
+    if cls._debug and cls.instance_max_retry:
       try:
-      cls.logger.debug("Starting setUpClass %s", cls)
+        cls.slap.waitForInstance(max_retry=cls.instance_max_retry - 1)
+      except SlapOSNodeCommandError:
+        cls.slap.waitForInstance(debug=True)
+    else:
+      cls.slap.waitForInstance(
+          max_retry=cls.instance_max_retry, debug=cls._debug)
+
+  @classmethod
+  def _setUpClass(cls):
     cls.slap.start()
     cls.logger.debug(
         "Formatting to remove old partitions XXX should not be needed because we delete ..."
@@ -391,20 +400,7 @@ class SlapOSInstanceTestCase(unittest.TestCase):
 
     # slapos node instance
     cls.logger.debug("Waiting for instance")
-      # waitForInstance does not tolerate any error but with instances,
-      # promises sometimes fail on first run, because services did not
-      # have time to start.
-      # To make debug usable, we tolerate instance_max_retry-1 errors and
-      # only debug the last.
-      if cls._debug and cls.instance_max_retry:
-        try:
-          cls.slap.waitForInstance(max_retry=cls.instance_max_retry - 1)
-        except SlapOSNodeCommandError:
-          cls.slap.waitForInstance(debug=True)
-      else:
-        cls.slap.waitForInstance(
-            max_retry=cls.instance_max_retry, debug=cls._debug)
-
+    cls.waitForInstance()
     # expose some class attributes so that tests can use them:
     # the main ComputerPartition instance, to use getInstanceParameterDict
     cls.computer_partition = cls.requestDefaultInstance()
@@ -412,7 +408,17 @@ class SlapOSInstanceTestCase(unittest.TestCase):
     # the path of the instance on the filesystem, for low level inspection
     cls.computer_partition_root_path = os.path.join(
         cls.slap._instance_root, cls.computer_partition.getId())
-      cls.logger.debug("setUpClass done")
+
+  @classmethod
+  def setUpClass(cls):
+    """Request an instance.
+    """
+    cls.logger.debug("Starting setUpClass %s", cls)
+    cls._instance_parameter_dict = cls.getInstanceParameterDict()
+    snapshot_name = "{}.{}.setUpClass".format(cls.__module__, cls.__name__)
+
+    try:
+      cls._setUpClass()
     except BaseException:
       cls.logger.exception("Error during setUpClass")
       cls._storeSystemSnapshot(snapshot_name)
@@ -421,6 +427,7 @@ class SlapOSInstanceTestCase(unittest.TestCase):
       raise
     else:
       cls._storeSystemSnapshot(snapshot_name)
+    cls.logger.debug("setUpClass done")
 
   @classmethod
   def tearDownClass(cls):