Makefile: ignore missing files during tests

[kemenaran]

When we test the target, md5sum currently prints a warning for each missing file.

azle.gbc: OK
md5sum: stat 'azle-r1.gbc': No such file or directory
md5sum: stat 'azle-r2.gbc': No such file or directory
md5sum: stat 'azlg.gbc': No such file or directory
md5sum: stat 'azlg-r1.gbc': No such file or directory
md5sum: stat 'azlj.gbc': No such file or directory
md5sum: stat 'azlj-r1.gbc': No such file or directory
md5sum: stat 'azlj-r2.gbc': No such file or directory
md5sum: stat 'azlf.gbc': No such file or directory
md5sum: stat 'azlf-r1.gbc': No such file or directory

Instead the make test target should ignore the missing files.

Implementation

md5sum seems to have an --ignore-missing option, but it doesn't seem to work on macOS. Maybe we'll have to re-implement a small md5sum from scratch.

[kemenaran]

A (rather old) simple re-implementation of md5sum in Python:

https://sebest.github.io/post/a-quick-md5sum-equivalent-in-python/

The best thing may be to adapt this for python3, and then to always use our script (rather than an OS-dependent version).