coverage: regression in texinfo "make mostlyclean" with $(SUBDIRS)

coverage: regression in texinfo "make mostlyclean" with $(SUBDIRS)

After commit v1.12.1-91-g205c757 of 2012-06-20, "texi: require
Texinfo >= 4.9, related enhancements", the presence of a Texinfo manual
'manual.texi' in a subdir (say 'doc/') of a package using a recursive
make setup would cause "make distcheck" to fail, due to the presence of
the 'manual.t2d/' directory created by texi2dvi.  That directory would
not be correctly removed because the 'mostlyclean' rule would run, from
within the 'doc/' sub-directory, "rm -rf doc/manual.t2d", instead of
the expected (and correct) "rm -rf manual.t2d".

Reported by Jim Meyering:
<http://lists.gnu.org/archive/html/automake-patches/2012-06/msg00203.html>

* t/txinfo21.sh: Enhance to expose the issue.
* t/txinfo-no-clutter.sh: Likewise, and other miscellaneous improvements.

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