Fix test_rm.py::test_secure_delete_capability
Fix test_rm.py::test_secure_delete_capability

When testing what happens when the shred command is not found, the
PATH environment variable shouldn't contain any location where it
could ever exist, as just happened in the upgrade from Xenial to
Focal. The test script also had a typo.

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop