ext4/053: test changing test_dummy_encryption on remount

ext4/053: test changing test_dummy_encryption on remount

The test_dummy_encryption mount option isn't supposed to be settable or
changeable via a remount, so add test cases for this.  This is a
regression test for a bug that was introduced in Linux v5.17 and fixed
in v5.19-rc3 by commit 85456054e10b ("ext4: fix up test_dummy_encryption
handling for new mount API").

Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>