test defs: really make them "set -e" clean

test defs: really make them "set -e" clean

Motivated by a spurious failure of test `instsh3-w.shtst' on
Solaris 10.

* tests/defs: Make really "set -e" clean, to avoid spurious
failures in case this file is sourced by a test script that has
already set the `errexit' flag.  To be sure we don't regress,
move the `set -e' call much earlier, and make the comment telling
about the expected "set -e" cleanliness more loud and outstanding.