Tests: normalize use of the 'errexit' shell flag.

Tests: normalize use of the 'errexit' shell flag.

* tests/maken3.test (check_targets): Remove redundant call to
'set -e'.
* tests/maken4.test: Likewise.
* tests/ansi5.test: Call 'set -e' just after './defs' has been
sourced.
* tests/ansi6.test: Likewise.
* tests/ansi7.test: Likewise.
* tests/cond16.test: Likewise.
* tests/cond17.test: Likewise.
* tests/cond18.test: Likewise.
* tests/cond19.test: Likewise.
* tests/cond20.test: Likewise.
* tests/cond21.test: Likewise.
* tests/instdat2.test: Likewise.
* tests/instdir-texi.test: Likewise.
* tests/parallel-tests3.test: Likewise.
* tests/remake1a.test: Likewise.
* tests/ccnoco.test: Likewise, and add trailing `:' command.
* tests/comment4.test: Likewise.
* tests/gcj4.test: Likewise.
* tests/nodist2.test: Likewise.
* tests/nodist3.test: Enable 'errexit' shell flag (this should
have been done in commit v1.11-248-g317e17b, but the relevant
hunk has been forgotten somehow).
* tests/output.test: Likewise.
* tests/gnits2.test: Likewise, and display captured stderr to
script's stderr, not to script's stdout.
* tests/gnits3.test: Likewise.  Also, prefer 'cat' over 'echo'
to append to Makefile.am, and really check that the exit status
of "make installcheck" indicates failure.