crypto: testmgr - Hide ENOENT errors better

The previous patch removed the ENOENT warning at the point of
allocation, but the overall self-test warning is still there.

Fix all of them by returning zero as the test result.  This is
safe because if the algorithm has gone away, then it cannot be
marked as tested.

Fixes: 4eded6d1 ("crypto: testmgr - Hide ENOENT errors")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>