Workaround bugs exposed by test_context changes

Workaround bugs exposed by test_context changes

Bugs exposed by 61720a0:

 - test_context --client-name=... --mech-type=ntlm ... fails;

 - gss_acquire_cred() with desired_mech=NTLM and
   desired_name==GSS_C_NO_NAME fails;

 - gss_init_sec_context() with non-default cred handle calls the
   mechanism even when the given cred handle has no element for the
   requencet mechanism.

tests/gss/check-ntlm works by accident: gss_acquire_cred() with
desired_mechs==GSS_C_NO_OID_SET succeeds mostly because there are
Kerberos credentials available, and then the subsequent
gss_init_sec_context() call works because of the third bug described
above.