cli/command/system: remove uses of pkg/errors in tests

While there may be reasons to keep pkg/errors in production code,
we don't need them for these tests.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/system/prune_test.go

@@ -2,6 +2,7 @@ package system
 
 import (
 	"context"
+	"errors"
 	"io"
 	"testing"
 
@@ -10,7 +11,6 @@ import (
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/api/types/network"
-	"github.com/pkg/errors"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
 )