gomod: run go work sync to sync modules

This commit is the result of running `go work sync` and `go mod tidy`
that syncs module versions for all submodules.

cert/go.mod

@@ -3,6 +3,8 @@ module github.com/lightningnetwork/lnd/cert
 go 1.16
 
 require (
-	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/stretchr/testify v1.5.1
+	github.com/kr/pretty v0.3.0 // indirect
+	github.com/rogpeppe/go-internal v1.9.0 // indirect
+	github.com/stretchr/testify v1.8.2
+	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 )