Move test code and metadata to tests-extension directory

Relocates all test-related code into the isolated test module:

Test code moved:
- test/extended/main.go → test/extended/tests-extension/main.go
- test/extended/apiserver/* → test/extended/tests-extension/apiserver/*
- cmd/openshift-apiserver-tests-ext → test/extended/tests-extension/cmd/

Metadata moved:
- test/.openshift-tests-extension/ → test/extended/tests-extension/.openshift-tests-extension/

This consolidates all test extension components (code, binary entry point,
and test metadata) into a single self-contained directory that uses its
own go.mod for dependency management.

The test binary and metadata now live alongside the test code in
test/extended/tests-extension/, making the test module completely
independent from the operator codebase.

Author: gangwgr

test/extended/README.md

@@ -21,8 +21,8 @@ import (
 	"github.com/spf13/cobra"
 
 	// The import below is necessary to ensure that the OAS operator tests are registered with the extension.
-	_ "github.com/openshift/openshift-apiserver/test/extended"
-	_ "github.com/openshift/openshift-apiserver/test/extended/apiserver"
+	_ "github.com/openshift/openshift-apiserver/test/extended/tests-extension"
+	_ "github.com/openshift/openshift-apiserver/test/extended/tests-extension/apiserver"
 )
 
 func main() {