and CFF retain-gids fixes

[skip ci]

modified subset_test_suite.py to apply desubroutinize profiles to CFF only (not to TT)

Disabled FDSelect serialization to format 0 for compatibility with fonttools

derived "expected" subset fonts from fonttools then manually tweaked further so they resemble hb-subset output

[subset] Fix checksum clearning in subset test diff. Add an updated expected file (has cmap4 restored).

Add basic directory structure for subsetter integration tests. Plus a utility for generating expected output files.