Update NodeToString Tests in Painless

[jdconrad]

The NodeToString tests are no longer valid with the current Painless refactorings. However, since there are still some major upcoming structural changes to the user tree nodes, the tests are disabled for now. Once the majority of the structural changes are completed, the tests need to be updated.

Relates to #49869

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[jdconrad]

This is no longer relevant as toString has been removed from the project in this way.