test: Properly clean up after stratis test failures

If TestStorageStratis.testBasic failed at a point when stratis
filesystems were mounted, the `pool stop` and `pool destroy` commands
would both fail, with "low-level ioctl error due to nix error" [sic] and
"filesystems remaining on pool" respectively. This also prevented the
loop devices from being cleaned up.

This broke the subsequent tests very hard, so that retries and testCLI
would all fail.

test/verify/check-storage-stratis

@@ -65,6 +65,8 @@ class TestStorageStratis(storagelib.StorageCase):
         self.addCleanup(exe,
                         "stratis report | jq -r '.pools[] | .name' |"
                         f"xargs -n1 --no-run-if-empty stratis pool stop {self.stop_type_opt}")
+        self.addCleanup(exe,
+                        "mount | grep mapper/stratis | awk '{print $1}' | xargs --no-run-if-empty umount")
 
     def testBasic(self):
         m = self.machine