tests: better name for a texinfo test

* t/txinfo21b: Rename ...
* t/txinfo-many-output-formats-vpath.sh: ... like this.  This matches
the similar rename "txinfo21" -> "txinfo-many-output-formats" done
for sister test 't/txinfo21.sh' in the master branch (rename that has
already been merged in the 'ng/master' branch.

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

diff --git a/t/txinfo21b.sh b/t/txinfo-many-output-formats-vpath.sh
similarity index 100%
rename from t/txinfo21b.sh
rename to t/txinfo-many-output-formats-vpath.sh