tests: Fix race condition in profile locking test.

tests: Fix race condition in profile locking test.

Previously, there was a possibility that "guix install emacs" would grab
the lock before "guix package -m $module_dir/manifest.scm".  When that
happened, the test would start building Emacs and all its dependencies,
which could take a while and some disk space.

This is a followup to b1fb663404894268b5ee92c040f12c52c0bee425.

* tests/guix-package.sh: In profile locking test, emit
"$module_dir/ready" from the manifest and wait for it to exist before
running "guix install emacs".