subdirs: enhance coverage, tweak and rename few tests

subdirs: enhance coverage, tweak and rename few tests

* t/subdir5.sh: Rename ...
* t/subdir-add-pr46.sh: ... like this.
* t/subdir8.sh: Rename ...
* t/subdir-add2-pr46.sh: ... like this.
* t/cond2.sh: Rename ...
* t/subdir-cond-err.sh: ... like this.
* t/subdir9.sh: Rename ...
* t/subdir-with-slash.sh: ... like this; improve heading comments.
* t/subcond.sh: Rename ...
* t/subdir-cond-gettext.sh: ... like this; improve m4 quoting.
* t/subcond2.sh: Rename ...
* t/subdir-am-cond.sh: ... like this; adjust heading comments.
* t/subcond3.sh: Rename ...
* t/subdir-ac-subst.sh: ... like this; adjust heading comments.
* t/subdir2.sh: Rename ...
* t/subdir-subsub.sh: ... like this; enhance so that it also runs
'./configure' and 'make'.
* t/subdir-order.sh: New test, check that the $(SUDBIRS) entries
are processed in the order they are specified.
* doc/automake.texi: Adjust references to tests.

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