Move testkit under test (#2133)

This change places `testkit` where it belongs, avoiding shipping it
with the rest of the codebase.

This might seem a trivial, but binary analysis tools might flag
ToolHive binaries as insecure because of that.

Author: blkt

pkg/authz/middleware_test.go

@@ -19,9 +19,9 @@ import (
 	"github.com/stacklok/toolhive/pkg/auth"
 	"github.com/stacklok/toolhive/pkg/logger"
 	mcpparser "github.com/stacklok/toolhive/pkg/mcp"
-	"github.com/stacklok/toolhive/pkg/testkit"
 	"github.com/stacklok/toolhive/pkg/transport/types"
 	"github.com/stacklok/toolhive/pkg/transport/types/mocks"
+	"github.com/stacklok/toolhive/test/testkit"
 )
 
 func TestMiddleware(t *testing.T) {

pkg/mcp/tool_middleware_test.go

@@ -9,7 +9,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/stacklok/toolhive/pkg/testkit"
+	"github.com/stacklok/toolhive/test/testkit"
 )
 
 func TestNewListToolsMappingMiddleware_Scenarios(t *testing.T) {