* tests/init.sh: New test framework from Gnulib.
* tests/basic.sh: New test.
* Makefile.am (TESTS): Add it.
(TEST_EXTENSIONS): Add '.sh'.
(SH_LOG_COMPILER): Use 'pre-inst-env'.
(EXTRA_DIST): Add 'tests/init.sh'.
* build-aux/pre-inst-env.in: export $srcdir for shell tests.
