test: Convert interleaved migration failure to a warning. (#4301)

ietf/utils/test_runner.py

@@ -52,6 +52,7 @@ import tempfile
 import copy
 import factory.random
 import urllib3
+import warnings
 from urllib.parse import urlencode
 
 from fnmatch import fnmatch
@@ -663,7 +664,7 @@ class CoverageTest(unittest.TestCase):
                 break
         mixed = [ unreleased[i] for i in range(s+1,len(unreleased)) if unreleased[i][1] != unreleased[i-1][1] ]
         if len(mixed) > 1 and not self.runner.permit_mixed_migrations:
-            raise self.failureException('Found interleaved schema and data operations in unreleased migrations;'
+            warnings.warn('Found interleaved schema and data operations in unreleased migrations;'
                 ' please see if they can be re-ordered with all data migrations before the schema migrations:\n'
                 +('\n'.join(['    %-6s:  %-12s, %s (%s)'% (op, node.key[0], node.key[1], nm) for (node, op, nm) in unreleased ])))