Pull request: all: imp some tests

Merge in DNS/adguard-home from imp-tests to master

Squashed commit of the following:

commit 6171420068d66808f069f280a81e0c60e61e5ce3
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 25 17:43:36 2021 +0300

    aghnet: imp test msgs

commit ed2880ff03ab0e41a77f07aad2caf13bf5f4bd9f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 25 17:39:44 2021 +0300

    all: imp some tests

internal/agherr/agherr_test.go

@@ -104,7 +104,7 @@ func TestAnnotate(t *testing.T) {
 		require.NotNil(t, errPtr)
 
 		err := *errPtr
-		require.NotNil(t, err)
+		require.Error(t, err)
 
 		assert.Equal(t, wantMsg, err.Error())
 	})
@@ -117,7 +117,7 @@ func TestAnnotate(t *testing.T) {
 		}
 
 		err := f()
-		require.NotNil(t, err)
+		require.Error(t, err)
 
 		assert.Equal(t, wantMsg, err.Error())
 	})