test_fx: enable weak crypto

Now that `test_fx` checks 1DES keys, we need to call enable_weak_crypto on the context.

Without this fix, "`make check`" was failing with the following error:

```
=============================================
   Heimdal 1.6rc2: lib/krb5/test-suite.log
=============================================

# TOTAL: 24
# PASS:  23
# SKIP:  0
# XFAIL: 0
# FAIL:  1
# XPASS: 0
# ERROR: 0

.. contents:: :depth: 2

FAIL: test_fx
=============

lt-test_fx: krb5_crypto_init: Encryption type des-cbc-crc not supported
```