Enable usetesting in golangci.yml

[sivchari]

usetesting linter is automatically enabled when Go 1.24 is used.

This linter diagnose the calls that can be replaced by functions defined in testing package. e.g. os.Setenv -> testing.Setenv.

It is what we're going to do after https://github.com/kubernetes-sigs/cluster-api/pull/12128 will be merged since the diff is very large.

The issue aims to enable usetesting linter and the fix all diagnostics to pass all.

/help