test-framework-sh: init.sh: fix "make dist" failure

When using gnulib-tool's --with-tests option and any module that
depends on test-framework-sh, "make dist" would fail due to the
lack of init.sh *in lib/*.  The EXTRA_DIST += init.sh is required
in the gltests directory, and not in the gllib/ directory.
One way to work around that is to move the EXTRA_DIST += init.sh
from the primary module to the -tests one:
* modules/test-framework-sh-tests (EXTRA_DIST): Add init.sh here, ...
* modules/test-framework-sh (Makefile.am): ...not here.
Reported by Tom G. Christensen in
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/29519