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

(cherry picked from commit 142a3cee1e294cb01c0f39a731312ca24792fcee)
Merge pull request #92 from ktdreyer/test-fx-weak-crypto-1-6-branch

1.6 cherry-pick: test_fx: enable weak crypto