tests: avoid DOS line endings in PSK file

tests: avoid DOS line endings in PSK file

Using FILE * APIs for writing the PSK file results in translation from
UNIX to DOS line endings on Windows. When the crypto PSK code later
loads the credentials the stray \r will result in failure to load the
PSK credentials into GNUTLS.

Rather than switching the FILE* APIs to open in binary format, just
switch to the more concise g_file_set_contents API.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>