Testing with testify not compatible with Go 1.12

[taktv6]

Hi there,

in PR #572 I was asked to add some tests to my code.
I did that and I needed a way to compare structures.
github.com/stretchr/testify is IMHO a great and easy way of doing so.

Problem is: It's not compatible with Go 1.12 and older.
Unfortunately the netlink projects CI tries to build for 1.12, 1.13 and 1.14.

Please allow me 2 questions:

1. What's the projects strategy regarding support for older Go versions? (If it would be "last 3 versions" then 1.12 should be dropped and 1.15 should be added.)
2. Is it okay to use testify here or is there an alternative I should use?

Kind Regards
takt