tests: autogenerate list of wrapped tests for `lib/' shell scripts

tests: autogenerate list of wrapped tests for `lib/' shell scripts

* tests/gen-config-shell-tests: New script, generates distributed
makefile snippet `tests/config-shell-tests.am' to list all tests
that use the `get_shell_script' function, with names mangled to
use suffix `-w.shtst', in ...
* tests/Makefile.am (config_shell_tests): ... this macro, whose
definition has been consequently removed from Makefile.am.
(EXTRA_DIST): Distribute the new script.
($(srcdir)/config-shell-tests.am): Generate using the new script.
(include): Include the `config-shell-tests.am' fragment.
* bootstrap: Invoke `tests/gen-config-shell-tests' to generate
`tests/config-shell-tests.am'.
* tests/.gitignore: Ignore `config-shell-tests.am'.
* tests/gen-parallel-tests: Fixlet in heading comments.