and CFF retain-gids fixes

Catch missing imports and errors like #1520 and #1521

__E901,E999,F821,F822,F823__ are the "_showstopper_" [flake8](http://flake8.pycqa.org) issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

[skip ci]

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

Currently hb-subset always desubroutinizes CFF fonts.
Note that even with this option fonttools won't generate the expected subset CFF fonts. They were manually created & tweaked to match hb-subset output.

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

Suppress progress messages of ttx. This avoids comparing this output to the reference file which lets the test fail.

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.

Fixes #873

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

I broke this in c31fcf4c