test: fix coding style

A few '*' characters were were misaligned. Fix them.

Signed-off-by: Andreas Bofjall <andreas@gazonk.org>
build: add missing stringtree header

stringtree_test includes stringtree.h but it didn't list it in
Makefile.am. Fix it.

Signed-off-by: Andreas Bofjall <andreas@gazonk.org>