Tests: stop execution if can't unmount any filesystem.

test/unit/utils.py

@@ -72,12 +72,19 @@ def waitforsocket(port):
     pytest.fail('Can\'t connect to the 127.0.0.1:' + str(port))
 
 
-def findmnt():
+def check_findmnt():
     try:
-        out = subprocess.check_output(
+        return subprocess.check_output(
             ['findmnt', '--raw'], stderr=subprocess.STDOUT
         ).decode()
     except FileNotFoundError:
+        return False
+
+
+def findmnt():
+    out = check_findmnt()
+
+    if not out:
         pytest.skip('requires findmnt')
 
     return out