Enable `errexit' shell flag in some test scripts.

Enable `errexit' shell flag in some test scripts.

* tests/dash.test: Enable `errexit' shell flag, and related
changes.
* tests/defun.test: Likewise.
* tests/defun2.test: Likewise.
* tests/dejagnu.test: Likewise.
* tests/double.test: Likewise.
* tests/distcom2.test: Likewise.
* tests/empty2.test: Likewise.
* tests/empty3.test: Likewise.
* tests/empty4.test: Likewise.
* tests/exdir.test: Likewise.
* tests/ext.test: Likewise.
* tests/extra.test: Likewise.
* tests/extra2.test: Likewise.
* tests/extra3.test: Likewise.
* tests/extra4.test: Likewise.
* tests/flibs.test: Likewise.
* tests/fnoc.test: Likewise.
* tests/fo.test: Likewise.
* tests/instexec.test: Likewise.
* tests/ltdeps.test: Likewise.
* tests/nodep.test: Likewise.
* tests/nodepcomp.test: Likewise.
* tests/f90only.test: Likewise, and remove botched/obsoleted
comments and unecessary commands.
* tests/fonly.test: Likewise, and removed botched/obsolted
comments and unecessary commands.
* tests/discover.test: Likewise, and made stricter.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>