In test, validate output of editor actions

[JRegimbal]

For NeonCore.test.js, validate the output of each action against the schema as an extra check to make sure our code will not invalidate a valid MEI.

[JRegimbal]

Attempting to load xml.js (at least our branch of it) and use it is enough to cause the test environment to run out of heap memory.

[JoyfulGen]

@yinanazhou we think this issue has been dealt with, but could you confirm?

[jinh0]

From what I saw, we aren't validating our XML in NeonCore.test.js , and looking through git log on this file, nothing about validation. So this is something we should do.