legacy tests: migrate test5
legacy tests: remove test21 files
legacy tests: implement :source helper method.
legacy tests: migrate test104
legacy tests: migrate test25
legacy tests: migrate test66
legacy tests: migrate test67
legacy tests: migrate test33
legacy tests: migrate test43
legacy tests: migrate test51
legacy tests: migrate test75
legacy tests: migrate test101
legacy tests: migrate test26
legacy tests: migrate test105
Merge PR #1420 'Migrate legacy tests (2nd batch)'
Remove win16 references
Remove NeXTSTEP references
Remove Atari MiNT references
Remove OS/2 references

NOTE: Paul Slootman was removed from the top of os_unix.c but is still
credited somewhere in the docs dir...
Remove Windows 95 references