tests: Make t-edit-sign idempotent

* tests/gpg/t-edit-sign.c (test_key, test_key_fpr, import_key,
delete_key): New.
(main): Remove key_fpr. Import test key to sign. Delete test key after
test.

Using a dedicated test key for testing key signing, which is imported
before the test and deleted after the test, makes sure that the
keys in the test key ring don't change. This is important for stable
results of the key listing tests.

• GnuPG-bug-id: T6342