test: let StorageHelpers inherit from TestCase

We don't want to re-define addCleanup ourself so we simply inherit
unittest.TestCase and type everything which uses addCleanup.

Author: jelly

test/common/storagelib.py

@@ -19,6 +19,7 @@
 import os.path
 import re
 import textwrap
+import unittest
 from typing import Any
 
 from machine import testvm
@@ -29,7 +30,7 @@ def from_udisks_ascii(codepoints: list[int]) -> str:
     return ''.join(map(chr, codepoints[:-1]))
 
 
-class StorageHelpers:
+class StorageHelpers(unittest.TestCase):
     """Mix-in class for using in tests that derive from something else than MachineCase or StorageCase"""
     machine: testvm.Machine
     browser: Browser
@@ -70,7 +71,7 @@ def add_ram_disk(self, size: int = 50, delay: int | None = None):
 
         return dev
 
-    def add_loopback_disk(self, size=50, name=None):
+    def add_loopback_disk(self, size: int = 50, name: str | None = None) -> str:
         """Add per-test loopback disk
 
         The disk gets removed automatically when the test ends. This is safe for @nondestructive tests.
@@ -103,7 +104,7 @@ def add_loopback_disk(self, size=50, name=None):
 
         return dev
 
-    def add_targetd_loopback_disk(self, index, size=50):
+    def add_targetd_loopback_disk(self, index: str, size: int = 50) -> str:
         """Add per-test loopback device that can be forcefully removed.
         """
 
@@ -135,12 +136,12 @@ def force_remove_disk(self, device: str) -> None:
         # the removal trips up PCP and our usage graphs
         self.allow_browser_errors("direct: instance name lookup failed.*")
 
-    def addCleanupVG(self, vgname):
+    def addCleanupVG(self, vgname: str) -> None:
         """Ensure the given VG is removed after the test"""
 
         self.addCleanup(self.machine.execute, f"if [ -d /dev/{vgname} ]; then vgremove --force {vgname}; fi")
 
-    def addCleanupMount(self, mount_point):
+    def addCleanupMount(self, mount_point: str) -> None:
         self.addCleanup(self.machine.execute,
                         f"if mountpoint -q {mount_point}; then umount {mount_point}; fi")