Add unit test for func RenderInstanceGroupSubnets in instancegroup.go
Add unit test for func RenderInstanceGroupSubnets in instancegroup.go

Signed-off-by: Guangwen Feng <98e12d19818083ae8a98303786e6a151211377ea@cn.fujitsu.com>

kubernetes/kops,robinpercy/kops,robinpercy/kops,justinsb/kops,justinsb/kops,robinpercy/kops,justinsb/kops,kubernetes/kops,robinpercy/kops,kubernetes/kops