virt-libguestfs: add 'no nic_custom' to tests.cfg

When you execute a libguestfs testcase, there will be 2 testcases running:
1. ***.raw.ide.smp2.rtl8139.****
2. ***.raw.ide.smp2.nic_custom.****
Now add 'no nic_custom' to filter the second case

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>