build: avoid test failure when sasl was not compiled in

build: avoid test failure when sasl was not compiled in

On systems without cyrus-sasl-devel available (I happened to be
in that situation on my FreeBSD testing), this test fails rather
miserably:

TEST: libvirtdconftest
      .....!!!!!!...!!!!!!!!!!!!!!!!!!!!!!!!!  39  FAIL
FAIL: libvirtdconftest

with verbose output showing things like:

39) Test corruption                                          ... libvir: Config File error : unsupporeted configuration: remoteReadConfigFile: /usr/home/dummy/libvirt/tests/../daemon/libvirtd.conf: auth_tcp: unsupported auth sasl

* tests/libvirtdconftest.c (testCorrupt): Avoid failure when sasl
is missing.