Context

This PR adds the missing unit test case for XMLTokener.

Test Plan

mvn -Dtest=XMLTokenerTest test

...
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running org.json.junit.XMLTokenerTest
[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.027 s -- in org.json.junit.XMLTokenerTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

Next Steps

1. Add more missing test cases to make this codebase more resilient.

What problem does this code solve?
Improves XMLTokener test coverage from 85% to 87%

Does the code still compile with Java6?
Yes

Risks
Low

Changes to the API?
No

Will this require a new release?
No

Should the documentation be updated?
No

Does it break the unit tests?
No, new unit tests were added

Was any code refactored in this commit?
No

Review status
APPROVED

Starting 3-day comment window

@stleary
Looks like you forgot to add the label. Can you please check?

Also when do you usually merge the PR?

3 days is the usual wait time. Don't often get comments, but it has happened.