tests: remove redundant `set -e', and other minor improvements

tests: remove redundant `set -e', and other minor improvements

* tests/dejagnu-absolute-builddir.test: Do not set the `errexit'
shell flag, as it is already set by `tests/defs'.
* tests/dejagnu-relative-srcdir.test: Likewise.
* tests/dejagnu-siteexp-append.test: Likewise.
* tests/dejagnu-siteexp-useredit.test: Likewise, and fix a typo
in comments.
* tests/dejagnu-siteexp-extend.test: Likewise, and prefer the
use of `$distdir' over hard-coded `$me-1.0'.  Also ...
(write_check_for): ... use `unindent' when writing from an
here-doc, for better code indentation.