[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/load_examples_test.py

@@ -0,0 +1,51 @@
+from superset import data
+from superset.cli import load_test_users_run
+from .base_tests import SupersetTestCase
+
+
+class SupersetDataFrameTestCase(SupersetTestCase):
+
+    def test_load_css_templates(self):
+        data.load_css_templates()
+
+    def test_load_energy(self):
+        data.load_energy()
+
+    def test_load_world_bank_health_n_pop(self):
+        data.load_world_bank_health_n_pop()
+
+    def test_load_birth_names(self):
+        data.load_birth_names()
+
+    def test_load_random_time_series_data(self):
+        data.load_random_time_series_data()
+
+    def test_load_country_map_data(self):
+        data.load_country_map_data()
+
+    def test_load_multiformat_time_series_data(self):
+        data.load_multiformat_time_series_data()
+
+    def test_load_paris_iris_geojson(self):
+        data.load_paris_iris_geojson()
+
+    def test_load_bart_lines(self):
+        data.load_bart_lines()
+
+    def test_load_multi_line(self):
+        data.load_multi_line()
+
+    def test_load_misc_dashboard(self):
+        data.load_misc_dashboard()
+
+    def test_load_unicode_test_data(self):
+        data.load_unicode_test_data()
+
+    def test_load_deck_dash(self):
+        data.load_long_lat_data()
+        data.load_flights()
+        data.load_sf_population_polygons()
+        data.load_deck_dash()
+
+    def test_load_test_users_run(self):
+        load_test_users_run()