Add and use keyfile_to_key_id() helper function

Add and use keyfile_to_key_id() helper function

When trying to extract the key ID from a key file name, some test code
incorrectly attempts to strip all leading zeros.  This breaks tests when
keys with ID 0 are generated.  Add a new helper shell function,
keyfile_to_key_id(), which properly handles keys with ID 0 and use it in
test code whenever a key ID needs to be extracted from a key file name.