Make it possible to run app tests with -count=n where n > 1

Clearly I didn't think through the effects of mutating the original test
case's store when I wrote this the first time.

Author: ahamlinman

internal/randomizer/app_test.go

@@ -232,13 +232,14 @@ var testCases = []struct {
 func TestMain(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.description, func(t *testing.T) {
-			app := NewApp("randomizer", tc.store)
+			store := tc.store.Clone()
+			app := NewApp("randomizer", store)
 			app.shuffle = slices.Sort
 
 			res, err := app.Main(context.Background(), tc.args)
 			tc.check(t, res, err)
 
-			if tc.expectedStore != nil && !reflect.DeepEqual(tc.store, tc.expectedStore) {
+			if tc.expectedStore != nil && !reflect.DeepEqual(store, tc.expectedStore) {
 				t.Errorf("unexpected store state\ngot:  %v\nwant: %v", tc.store, tc.expectedStore)
 			}
 		})

internal/randomizer/rndtest/rndtest.go

@@ -12,6 +12,18 @@ import (
 // strings. A nil Store will return errors for every operation.
 type Store map[string][]string
 
+// Clone returns a deep copy of the original store.
+func (s Store) Clone() Store {
+	if s == nil {
+		return nil
+	}
+	out := make(Store, len(s))
+	for k, v := range s {
+		out[k] = slices.Clone(v)
+	}
+	return out
+}
+
 // List implements randomizer.Store.
 func (s Store) List(_ context.Context) ([]string, error) {
 	if s == nil {