diff --git a/probert/tests/__init__.py b/probert/tests/__init__.py
index e69de29..9c1555c 100644
--- a/probert/tests/__init__.py
+++ b/probert/tests/__init__.py
@@ -0,0 +1,11 @@
+import shutil  # noqa: F401
+from unittest import IsolatedAsyncioTestCase
+from unittest.mock import patch
+
+
+class ProbertTestCase(IsolatedAsyncioTestCase):
+    def setUp(self):
+        which = patch("shutil.which")
+        self.m_which = which.start()
+        self.m_which.return_value = '/bin/false'
+        self.addCleanup(which.stop)
diff --git a/probert/tests/test_filesystem.py b/probert/tests/test_filesystem.py
index 6334af3..580322c 100644
--- a/probert/tests/test_filesystem.py
+++ b/probert/tests/test_filesystem.py
@@ -27,6 +27,7 @@
     get_swap_sizing,
     get_device_filesystem,
 )
+from probert.tests import ProbertTestCase
 
 
 def read_file(filename):
@@ -60,8 +61,9 @@ async def test_expected_output_vg(self):
         assert expected == await get_swap_sizing(device)
 
 
-class TestFilesystem(IsolatedAsyncioTestCase):
+class TestFilesystem(ProbertTestCase):
     def setUp(self):
+        super().setUp()
         self.device = Mock()
         self.device.device_node = random_string()
 
@@ -196,17 +198,18 @@ async def test_get_device_filesystem_sizing_ext4_no_min(self):
             actual = await get_device_filesystem(self.device, True)
             self.assertEqual(expected, actual)
 
-    @patch('probert.filesystem.shutil.which')
-    async def test_ntfsresize_not_found(self, which):
-        which.return_value = None
+
+class TestFilesystemToolNotFound(ProbertTestCase):
+    def setUp(self):
+        super().setUp()
+        self.device = Mock()
+        self.m_which.return_value = None
+
+    async def test_ntfsresize_not_found(self):
         self.assertEqual(None, await get_ntfs_sizing(self.device))
 
-    @patch('probert.filesystem.shutil.which')
-    async def test_dumpe2fs_not_found(self, which):
-        which.return_value = None
+    async def test_dumpe2fs_not_found(self):
         self.assertEqual(None, await get_dumpe2fs_info(self.device))
 
-    @patch('probert.filesystem.shutil.which')
-    async def test_resize2fs_not_found(self, which):
-        which.return_value = None
+    async def test_resize2fs_not_found(self):
         self.assertEqual(None, await get_resize2fs_info(self.device))
diff --git a/probert/tests/test_os.py b/probert/tests/test_os.py
index a7b5f19..823d6e6 100644
--- a/probert/tests/test_os.py
+++ b/probert/tests/test_os.py
@@ -14,13 +14,13 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import subprocess
-from unittest import IsolatedAsyncioTestCase
 from unittest.mock import patch
 
 from probert.os import probe, _parse_osprober, _run_os_prober
+from probert.tests import ProbertTestCase
 
 
-class TestOsProber(IsolatedAsyncioTestCase):
+class TestOsProber(ProbertTestCase):
     def tearDown(self):
         _run_os_prober.cache_clear()
 
@@ -154,3 +154,12 @@ async def test_run_once(self, run):
         self.assertEqual({}, await probe())
         self.assertEqual({}, await probe())
         run.assert_called_once()
+
+
+class TestOsProberNotFound(ProbertTestCase):
+    def setUp(self):
+        super().setUp()
+        self.m_which.return_value = None
+
+    async def test_os_prober_not_found(self):
+        self.assertEqual(None, _run_os_prober())