For the API tests, output the test programs at $(TOP_BUILDDIR) so that
the freshly-built DLLs will be available for the test programs.  For
those that are run through the Python wrapper scripts, use
${PYTHON_EXECUTABLE} instead of plain 'python' in case the Python
interpreter is not in the PATH.

[subset] zero glyf and loca memory before writing so pads are clean. Test just the part of maxp we care about

[subset] sketch out dropping in-glyf instructions. Sometimes yields differnet size glyphs than fonttools, possibly due to padding not being corrected

New API:
- hb_set_previous()
- hb_set_previous_range()

To signify that unlike rest of library, returns nullptr on failure.