[test] Speed-up subset tests by saving TTX dump

Speed-up subset tests by saving TTX dump of expected output instead of
generating it each time the tests are run.

Cuts down meson test --suite=subset on my system from:
real	0m38.977s
user	1m12.024s
sys	0m10.547s

to:
real	0m22.291s
user	0m44.548s
sys	0m9.221s

Part of https://github.com/harfbuzz/harfbuzz/issues/3089