Allow test runners to wrap contexts

Let the test runners choose the namespaces and
wrap the contexts. This allows the test suite to create
multiple contexts without worrying about namespacing
or leasing in the contexts.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

namespaces/context.go

@@ -1,11 +1,11 @@
 package namespaces
 
 import (
+	"context"
 	"os"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/pkg/errors"
-	"golang.org/x/net/context"
 )
 
 const (