tests: fix bashism to load helpers.sh
`source` isn't available at POSIX sh.
#!/bin/sh
cat <<EOF >> $HGRCPATH
[extensions]
schemes=
[schemes]
l = http://localhost:$HGPORT/
parts = http://{1}:$HGPORT/
z = file:\$PWD/
EOF
hg init test
cd test
echo a > a
hg ci -Am initial
hg serve -n test -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log
cat hg.pid >> $DAEMON_PIDS
hg incoming l://
echo % check that {1} syntax works
hg incoming --debug parts://localhost | sed 's/[0-9]//g'
echo % check that paths are expanded
PWD=`pwd` hg incoming z://
echo % errors
cat errors.log