tests: fix bashism to load helpers.sh
`source` isn't available at POSIX sh.
% fail
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
inotify-client: could not start inotify server: child process failed to start
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
% inserve
% status
? hg.pid