common/encrypt: move constant test key to common code

For some encryption tests it's helpful to always use the same key so
that the test's output is always the same.

generic/580 already defines such a key, so move it into common/encrypt
so that other tests can use it too.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>