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: I4fbfa8fba84b11c4e30e4db9a0c358db1e8c94f1

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__))
 _actions = os.path.abspath(os.path.join(_path, '../actions'))
@@ -33,3 +34,15 @@ _add_path(_hooks)
 _add_path(_charmhelpers)
 _add_path(_unit_tests)
 _add_path(_scripts)
+
+# 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()