tests/test-empty

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

#!/bin/sh

hg init a
cd a
hg log
hg grep wah
hg manifest
hg verify
ls .hg
ls .hg/store

cd ..
hg clone a b
cd b
hg verify
ls .hg
ls .hg/store