tests: Run the luks tests in test-crypto-block only if encryption is available

The test-crypto-block currently fails if encryption has not been
compiled into QEMU:

TEST: tests/test-crypto-block... (pid=22231)
  /crypto/block/qcow:                                                  OK
  /crypto/block/luks/default:
  Unexpected error in qcrypto_pbkdf2() at qemu/crypto/pbkdf-stub.c:41:
FAIL
GTester: last random seed: R02Sbbb5b6f299c6727f41bb50ba4aa6ef5c
(pid=22237)
  /crypto/block/luks/aes-256-cbc-plain64:
  Unexpected error in qcrypto_pbkdf2() at qemu/crypto/pbkdf-stub.c:41:
FAIL
GTester: last random seed: R02S3e27992a5ab4cc95e141c4ed3c7f0d2e
(pid=22239)
  /crypto/block/luks/aes-256-cbc-essiv:
  Unexpected error in qcrypto_pbkdf2() at qemu/crypto/pbkdf-stub.c:41:
FAIL
GTester: last random seed: R02S51b52bb02a66c42d8b331fd305384f53
(pid=22241)
FAIL: tests/test-crypto-block

So run the luks test only if the required encryption support is available.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>