Improve, extend and tweak tests on Texinfo support.

Improve, extend and tweak tests on Texinfo support.

* tests/instdir-texi.test: Add a call to `ls -l' after that to
`make', for debugging.  When looking for required tools, do not
redirect the output of "$tool --help" to /dev/null, and do not
uselessly run it in a subshell.
* tests/txinfo.test: Rewritten to run autoconf, ./configure and
make.  All checks moved into Makefile.am.
* tests/txinfo8.test: Likewise, and modernize the generated
configure.in.
* tests/txinfo2.test: Moved checks into Makefile.am, and other
minor improvements.
* tests/txinfo5.test: Enable `errexit' shell flag, and related
changes.  Add trailing `:' command.
* tests/txinfo6.test: Likewise, and make grepping of generated
Makefile.in stricter.
* tests/txinfo7.test: Enable `errexit' shell flag, and related
changes.  Add trailing `:' command.  Do not add unnecessary stuff
to Makefile.am.
* tests/txinfo9.test: Verify that more targets which are expected
to be generated only once really are.  Make grepping less strict,
to avoid exposing too much internal details.  More minor changes.
* tests/txinfo16.test: Add trailing `:'.  Prefer cat over echo
for appending to configure.in.  Updated/fixed heading comments.
* tests/txinfo23.test: Likewise, and extended a little by making
it check that no info file is created in the $(srcdir).
* tests/txinfo24.test: Likewise.
* tests/txinfo25.test: Likewise.
* tests/txinfo18.test: Add trailing `:'.  Prefer cat over echo
for appending to configure.in.  Also, check that index files are
cleaned also by "make clean", not only by "make distclean".
* tests/txinfo22.test: Prefer `$me' over hard-coded test name,
and added trailing `:' command.  This testcase also used to check
that automake ignores in-line comments when using variables, but
preserves them in the output; these checks (added in commit
"Release-1-7f-4-g9177ef8") do not really pertain to this test,
so they have been moved ...
* tests/comments-in-var-defn.test: ... into this new test.
* tests/txinfo4.test: Escape literal dots in grep regexps.  Add
trailing `:' command.
* tests/txinfo29.test: Likewise.  Relax grepping of generated
Makefile.in w.r.t. whitespaces.  Prefer `cat' over `echo' to
append to configure.in.
* tests/txinfo3.test: Likewise.
* tests/vtexi.test: Improve grepping of Makefile.in (sometimes
make it stricter, sometimes laxer).  Move `set -e' setting just
after the inclusion of ./defs.  De-uglify a sed command.  Other
minor cosmetic improvements.
* tests/vtexi2.test: Make grepping of Makefile.in stricter.  Add
trailing `:' command.
* tests/vtexi3.test: New test on version.texi support.
* tests/vtexi4.test: Likewise.
* tests/Makefile.am (TESTS): Updated.