Let Treehugger run CtsSaxTestCases for changes to luni javax.xml.

This test has historically been extremely stable (I don't recall
it ever failing) and provides little coverage, but at the same
time it completes quickly (20sec for atest including test setup,
net test runtime is only 1msec). It seems reasonable to run it for
the rare CLs that touch javax.xml.

Test: (croot libcore/luni/src/main/java/javax/xml ; atest)

Change-Id: Ifafeb09c9f808250643a1970c43e9f896598f861