tests: DRYing out `tests/sh.test'.

tests: DRYing out `tests/sh.test'.

* Makefile.am (libtool_m4): Canonical location of libtool.m4 file.
(lt_aclocal_m4_deps): Use it.
(check_scripts): New variable for list of scripts to be checked
by `tests/sh.test'.
(legacy_tests_edit): New sed script to substitute `libtool_m4'
and `check_scripts' into `tests/defs'.
(tests/defs.in): Use it.
* tests/defs.m4sh (scripts): Calculate absolute paths of each
entry in `check_scripts' before changing directories.
(libtool_m4): Save location of `libtool.m4'.
(macro_dir, aux_dir): Removed. No longer required by any test.
* tests/sh.test: Remove loop around just `$macro_dir/libtool.m4'
in favour of a single reference to `$libtool_m4' from above.