test_fx: enable weak crypto

Now that test_fx checks 1DES keys, we need to call allow_weak_crypto on
the test's context.

Without this fix, "make check" was failing with the following error:

  lt-test_fx: krb5_crypto_init: Encryption type des-cbc-crc not
  supported
Merge pull request #91 from ktdreyer/test-fx-weak-crypto

test_fx: enable weak crypto