tests: some enhancements to texinfo tests (from Automake-NG)
* t/txinfo-many-output-formats.sh: Backport improvements, almost verbatim,
from Automake-NG commit '
v1.13-768-gb434acc' (plus minor tweaks for the
sake of non-GNU make implementation).
* t/txinfo-many-output-formats-vpath.sh: New test, backported almost
verbatim from that same Automake-NG commit (again, with minor tweaks for
the sake of non-GNU make implementations).
* t/lsit-of-tests.mk: Adjust.
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>