Some targets such as LIO support base64 encoded CHAP_R values. Test this
by intercepting hex-encoded CHAP_R responses and converting the value to
base64.
This will fail on targets which don't support base64; we should probably
skip the test instead.
base64 encoding is performed using the gnutls_base64_encode2() function,
so the test is only present for gnutls-enabled builds.
Signed-off-by: David Disseldorp <ddiss@suse.de>