[subset] Remove test-subset, testing is planned to be done at the table level here with test/subset covering the complete subsetting operation.

Since this is a convenience library, it's not actually linked. Or so I think.
Hoping this might fix this Appveyor failure:

https://ci.appveyor.com/project/harfbuzz/harfbuzz/build/job/9884jyb5vryja9pw

Making all in hb-ucdn
make[4]: Entering directory '/c/projects/harfbuzz/src/hb-ucdn'
  CC       libhb_ucdn_la-ucdn.lo
  GEN      libhb-ucdn.la
/bin/sh: libhb_ucdn_la-ucdn.lo: command not found
make[4]: *** [Makefile:404: libhb-ucdn.la] Error 127
make[4]: Leaving directory '/c/projects/harfbuzz/src/hb-ucdn'
make[3]: *** [Makefile:2548: all-recursive] Error 1
make[3]: Leaving directory '/c/projects/harfbuzz/src'
make[2]: *** [Makefile:1382: all] Error 2
make[2]: Leaving directory '/c/projects/harfbuzz/src'
make[1]: *** [Makefile:486: all-recursive] Error 1
make[1]: Leaving directory '/c/projects/harfbuzz'
make: *** [Makefile:418: all] Error 2

Fixes https://github.com/harfbuzz/harfbuzz/pull/776

This one might do it...

I broke this in c31fcf4c

If we don't have visibility, make NullPool static.

Better fix for https://github.com/harfbuzz/harfbuzz/pull/775