errors: fix package example

The example in example_test.go requires that the whole file be
displayed; the addition of ExampleAs meant that only the body of the
package example function was shown, rather than the surrounding context.

This change moves ExampleAs to the file wrap_test.go file, restoring the
package example to its former glory.

Update #31716

Change-Id: Id0ea77bc06023b239a63c1d6a7c8b3c1dae91ce9
Reviewed-on: https://go-review.googlesource.com/c/go/+/179737
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Jean de Klerk <deklerk@google.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/errors/example_test.go

@@ -5,9 +5,7 @@
 package errors_test
 
 import (
-	"errors"
 	"fmt"
-	"os"
 	"time"
 )
 
@@ -34,17 +32,3 @@ func Example() {
 	}
 	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
 }
-
-func ExampleAs() {
-	if _, err := os.Open("non-existing"); err != nil {
-		var pathError *os.PathError
-		if errors.As(err, &pathError) {
-			fmt.Println("Failed at path:", pathError.Path)
-		} else {
-			fmt.Println(err)
-		}
-	}
-
-	// Output:
-	// Failed at path: non-existing
-}

src/errors/wrap_test.go

@@ -218,3 +218,17 @@ func (errorUncomparable) Is(target error) bool {
 	_, ok := target.(errorUncomparable)
 	return ok
 }
+
+func ExampleAs() {
+	if _, err := os.Open("non-existing"); err != nil {
+		var pathError *os.PathError
+		if errors.As(err, &pathError) {
+			fmt.Println("Failed at path:", pathError.Path)
+		} else {
+			fmt.Println(err)
+		}
+	}
+
+	// Output:
+	// Failed at path: non-existing
+}