fix(test): restore environment after parseWithEnv

Make sure tests don't have side-effects
potentially breaking other tests.

parse_test.go

@@ -44,12 +44,33 @@ func parseWithEnv(cmdline string, env []string, dest interface{}) (*Parser, erro
 	}
 
 	// split the environment vars
+	newEnv := map[string]string{}
+	oldEnv := map[string]string{}
 	for _, s := range env {
 		pos := strings.Index(s, "=")
 		if pos == -1 {
 			return nil, fmt.Errorf("missing equals sign in %q", s)
 		}
-		err := os.Setenv(s[:pos], s[pos+1:])
+
+		if orig, ok := os.LookupEnv(s[:pos]); ok {
+			oldEnv[s[:pos]] = orig
+		}
+
+		newEnv[s[:pos]] = s[pos+1:]
+	}
+
+	defer func() {
+		for key, _ := range newEnv {
+			if orig, ok := oldEnv[key]; ok {
+				_ = os.Setenv(key, orig)
+			} else {
+				_ = os.Unsetenv(key)
+			}
+		}
+	}()
+
+	for key, val := range newEnv {
+		err := os.Setenv(key, val)
 		if err != nil {
 			return nil, err
 		}