Update tests for addition and subtraction in same section.

Forward port of http://svn.zope.org/zc.buildout/trunk/src/zc/buildout/buildout.txt?rev=127336&view=diff&r1=127336&r2=127335&p1=zc.buildout/trunk/src/zc/buildout/buildout.txt&p2=/zc.buildout/trunk/src/zc/buildout/buildout.txt