efi_selftest: Update StrToFat() unit test after CP473 map extension

Test that Unicode code points which map to CP437 code points 1-31 are
converted to '_'. This ensures no FAT file names do not contain chars
which are control characters in other code pages (CP 1250 for example).

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>