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: I27979aaeec8040b8cd5c07e5ad12a6ca8544278c

unit_tests/__init__.py

@@ -14,6 +14,7 @@
 
 import os
 import sys
+from unittest.mock import patch
 
 _path = os.path.dirname(os.path.realpath(__file__))
 _root = os.path.abspath(os.path.join(_path, '..'))
@@ -27,3 +28,15 @@ def _add_path(path):
 _add_path(_path)
 _add_path('actions')
 _add_path('hooks')
+
+# 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()