[utils.py] gathering/refactoring into a "utils/" folder (#6095)

* [utils] gathering/refactoring into a "utils/" folder

Moving current utils.py into utils/core.py and moving other *util*
modules under this new "utils/" as well.

Following steps include eroding at "utils/core.py" and breaking it down
into smaller modules.

* Improve tests

* Make loading examples in scope for tests

* Remove test class attrs examples_loaded and requires_examples

tests/import_export_tests.py

@@ -4,23 +4,19 @@ import unittest
 
 from sqlalchemy.orm.session import make_transient
 
-from superset import db, utils
+from superset import db
 from superset.connectors.druid.models import (
     DruidColumn, DruidDatasource, DruidMetric,
 )
 from superset.connectors.sqla.models import SqlaTable, SqlMetric, TableColumn
 from superset.models import core as models
+from superset.utils import core as utils
 from .base_tests import SupersetTestCase
 
 
 class ImportExportTests(SupersetTestCase):
     """Testing export import functionality for dashboards"""
 
-    requires_examples = True
-
-    def __init__(self, *args, **kwargs):
-        super(ImportExportTests, self).__init__(*args, **kwargs)
-
     @classmethod
     def delete_imports(cls):
         # Imported data clean up
@@ -42,6 +38,7 @@ class ImportExportTests(SupersetTestCase):
     @classmethod
     def setUpClass(cls):
         cls.delete_imports()
+        cls.create_druid_test_objects()
 
     @classmethod
     def tearDownClass(cls):