test-convert-darcs: don't let $HOME/.darcs pollute the test environment

tests/test-convert-darcs

@@ -7,6 +7,7 b' echo "convert=" >> $HGRCPATH'
 echo 'hgext.graphlog =' >> $HGRCPATH
 
 DARCS_EMAIL='[e-mail]'; export DARCS_EMAIL
+HOME=do_not_use_HOME_darcs; export HOME
 
 echo % initialize darcs repo
 mkdir darcs-repo