[nightly] Make xml test non-voting in branch-1.2

HBASE-19354 moved branch-1.2 to use azul openjdk. Turns out zulu-7 does does not have a js script engine so jrunscript fails which causes the xml check to fail with

19:55:59 | -1 | xml | 0m 7s | The source tree has 60 ill-formed XML
19:55:59 | | | | file(s).

The xml.txt file has this in it:

script engine for language js can not be found

If I run jrunscript from zulu-7 on cmd-line, it does this:

$ $JAVA_HOME/bin/jrunscript
script engine for language js can not be found

For now, make the xml check non-voting in branch-1.2 so can see if we can get the nightly running.