upgrade to use testfixtures v3 (#11904)

* upgrade to use testfixtures v3

* simplify logic

* make vendor

* update per @lunny

* Update templates/repo/empty.tmpl

* Update templates/repo/empty.tmpl

Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/spf13/pflag/flag.go

@@ -57,9 +57,9 @@ that give one-letter shorthands for flags. You can use these by appending
 	var ip = flag.IntP("flagname", "f", 1234, "help message")
 	var flagvar bool
 	func init() {
-		flag.BoolVarP("boolname", "b", true, "help message")
+		flag.BoolVarP(&flagvar, "boolname", "b", true, "help message")
 	}
-	flag.VarP(&flagVar, "varname", "v", 1234, "help message")
+	flag.VarP(&flagval, "varname", "v", "help message")
 Shorthand letters can be used with single dashes on the command line.
 Boolean shorthand flags can be combined with other shorthand flags.
 
@@ -190,6 +190,18 @@ type Value interface {
 	Type() string
 }
 
+// SliceValue is a secondary interface to all flags which hold a list
+// of values.  This allows full control over the value of list flags,
+// and avoids complicated marshalling and unmarshalling to csv.
+type SliceValue interface {
+	// Append adds the specified value to the end of the flag value list.
+	Append(string) error
+	// Replace will fully overwrite any data currently in the flag value list.
+	Replace([]string) error
+	// GetSlice returns the flag value list as an array of strings.
+	GetSlice() []string
+}
+
 // sortFlags returns the flags as a slice in lexicographical sorted order.
 func sortFlags(flags map[NormalizedName]*Flag) []*Flag {
 	list := make(sort.StringSlice, len(flags))