Modernize, improve and extend tests `suffix*.test'.

Modernize, improve and extend tests `suffix*.test'.

* tests/suffix3.test: Enable `errexit' shell flag, and related
changes.
* tests/suffix4.test: Likewise.
* tests/suffix6.test: Likewise, and do not create a useless dummy
source file.
* tests/suffix7.test: Likewise.
* tests/suffix5.test: Enable `errexit' shell flag, and related
changes.  Make grepping of Makefile.in slighty stricter.
* tests/suffix.test: Enable `errexit' shell flag, and related
changes.  Also, do not redirect grep output to /dev/null, as this
might unmotivatedly hide useful information.
* tests/suffix2.test: Move setting of `errexit' shell flag earlier
in the script (just after inclusion of ./defs).  Use a more
idiomatic way to count text occurrences in Makefile.in with
grep.  Do not create useless dummy source files.
* tests/suffix10.test: Ensure verbose printing of captured make
stdout.  Minor cosmetic changes.
* tests/suffix8.test: Likewise.  Also, drop useless call to the
env(1) utility, and make grepping of make output stricter by using
$FGREP rather than plain grep.
* tests/suffix11.test: Likewise.
* tests/suffix12.test: Likewise.
* tests/suffix9.test: Prefet cat + here-doc over echo to append to
the `configure.in' stub.  Cosmetic changes.
* tests/suffix13.test: Cosmetic spacing change.

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