[test] Add batch mode to hb-subset and use it

time meson test --suite=subset down from:
real	0m22.822s
user	0m44.561s
sys	0m9.255s

to:
real	0m19.418s
user	0m38.171s
sys	0m3.587s

Does not seem to help much, but it is something.

Part of https://github.com/harfbuzz/harfbuzz/issues/3089
3 files changed