Pull request: all: rm var shadowing vol. 3

Updates #2803.

Squashed commit of the following:

commit ce711ae2e08c3714a4e0c6d177c02f7801c05fce
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:58:40 2021 +0300

    all: imp code, docs

commit e91094e461893c27eda879b33b5ed2c7085510df
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:42:14 2021 +0300

    all: rm var shadowing vol. 3

internal/aghtest/os_windows.go

@@ -13,7 +13,7 @@ import (
 )
 
 const (
-	maxRetryDur = 500 * time.Millisecond
+	maxRetryDur = 1000 * time.Millisecond
 	retryDur    = 5 * time.Millisecond
 )
 
@@ -56,7 +56,7 @@ func prepareTestDir(t *testing.T) (dir string) {
 			time.Sleep(retryDur)
 		}
 
-		assert.Nil(t, err)
+		assert.Nil(t, err, "after %s", time.Since(start))
 	})
 
 	return dir