Improve platform mocking

Patch out charmhelpers.osplatform.get_platform() and
charmhelpers.core.host.lsb_release() globally in the unit tests to
insulate the unit tests from the platform that the unit tests are being
run on.

Change-Id: I3ca0e47cc8cd0bc8475c0c4b0ef8d013d52401ed

unit_tests/__init__.py

@@ -15,6 +15,8 @@
 import os
 import sys
 
+from unittest.mock import patch
+
 _path = os.path.dirname(os.path.realpath(__file__))
 _actions = os.path.abspath(os.path.join(_path, '../actions'))
 _hooks = os.path.abspath(os.path.join(_path, '../hooks'))
@@ -29,3 +31,12 @@ def _add_path(path):
 _add_path(_actions)
 _add_path(_hooks)
 _add_path(_lib)
+
+# Patch out lsb_release() and get_platform() as unit tests should be fully
+# insulated from the underlying platform.  Unit tests assume that the system is
+# ubuntu jammy.
+patch('charmhelpers.osplatform.get_platform', return_value='ubuntu').start()
+patch('charmhelpers.core.host.lsb_release',
+      return_value={
+          'DISTRIB_CODENAME': 'jammy',
+      }).start()