grepping test/jaxp for @library found the following jaxp-specific test libraries:
/javax/xml/jaxp/functional
/javax/xml/jaxp/isolatedjdk
/javax/xml/jaxp/libs
/javax/xml/jaxp/unittest
at a glance, only jaxp/javax/xml/jaxp/libs/jaxp/library/ directory has some classes which might be considered as generic and suggested for moving to the top-level library. and it seems there is only one such classed remained – jaxp.library.SimpleHttpServer, which will be handled by JDK-8211979
I'd really appreciate someone of jaxp team to (dis)confirm that there are no other "generic purpose" test library classes within jaxp test suite.