Pull request: all: rm var shadowing, vol. 1

Updates #2803.

Squashed commit of the following:

commit 15fa64ad4fb7561c4b0542245f99869685643bbd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 14:55:28 2021 +0300

    util: imp code

commit c3b0563a44ccc98a5901df19174acdca8a350d62
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 14:38:54 2021 +0300

    all: rm var shadowing, vol. 1

internal/aghtest/os_windows.go

@@ -44,7 +44,7 @@ func prepareTestDir(t *testing.T) (dir string) {
 	t.Cleanup(func() {
 		start := time.Now()
 		for {
-			err := os.RemoveAll(dir)
+			err = os.RemoveAll(dir)
 			if err == nil {
 				break
 			}