Tests cannot be built from the source archive

[Szabolcs Horvát]

The tests cannot be built from the source archive at https://pngquant.org/releases.html

The following steps work from the cloned git repo (tag 2.14.1) but not from the source archive:

./configure
make
make test

The error for make test is

make: *** No rule to make target `test/test.o', needed by `test/test'.  Stop.

[Kornel]

I'll add them in a next release