fscrypt: make test_dummy_encryption use v2 by default

Since v1 encryption policies are deprecated, make test_dummy_encryption
test v2 policies by default.

Note that this causes ext4/023 and ext4/028 to start failing due to
known bugs in those tests (see previous commit).

Link: https://lore.kernel.org/r/20200512233251.118314-5-ebiggers@kernel.org
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>