Test infrastructure needed

[paullinnerud]

Build a Python test infrastructure to test built VTTCompile output against known good state.

Include open sourced Selawik font and use fonttools TTX to dump relevant tables then use built VTTCompile to rebuild all and dump relevant tables to compare with original.

[HinTak]

For comparison, it is useful to ignore checkSumAdjustment (because of the date modified) and
modified and many lines of the form /* VTT 6.35 compiler Wed Oct 20 21:11:18 2021 */.

I have been using vttdemo.ttf for testing so far, and we have just gotten to the point every switch with vttcompile gives identical results (caveat the three difference above) with vttshell. vttdemo does not have any cache tables? As the results of -b and -c are the same.

vttshell does run quite well on wine; in any case, I'd suggest getting mingw vttcompile comparison with vttshell on windows or wine. That covers gcc + vttcompile vs vc vttshell differences; then gcc vtcompile vs clang vttcompile, which would mostly warrant correct functionality where clang is the system compiler (macs...).