* Makefile.am (scripts/gash): Make sure the 'scripts' directory exists
in the build tree.
(SH_LOG_COMPILER): Call 'test.sh' from the source tree.
* test.sh: Change the default test shell from 'scripts/gash' to
'gash'; export a 'TEST_TMP' variable that tells tests where to store
temporary files; and make sure all output is written to the build
directory.
tests/50-redirect-append.sh: Use the 'TEST_TMP' variable.
tests/60-subst.sh: Ditto.