tests: Add option "allow-del" to run-threaded

* tests/run-threaded.c (main): Handle allow-del.
(allow-del): New. Variable to allow deletion of keys.
(import): Delete key after import if allow-del is set.
(delete_impres): Delete keys from an import result.
(delete_fpr): Delete a key by fingerprint.

This is intended to test write access and locking of the
keyring by repeatedly importing and deleting pubkeys.

It is an option because it might cause keyring corruption
etc. so it should be explicitly enabled.