tests: enhance and tweak some texinfo tests

tests: enhance and tweak some texinfo tests

* t/txinfo-subdir-pr343.sh: Move the tests checking that '.info.bak'
files in subdirs are not unduly distributed into ...
* t/txinfo-no-extra-dist.sh: ... in here.  Enhance a little while
at it.
* t/txinfo-other-suffixes.sh: Also check that the '.texinfo' suffix
is accepted and works.
* t/txinfo-setfilename-suffix-match.sh: Remove useless call to
autoconf, enhance a little.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>