efi_selftest: Add international characters test

UEFI relies entirely on unicode output, which actual fonts displayed on
the screen might not be ready for.

Add a test displaying some international characters, to reveal missing
glyphs, especially in our builtin fonts.
This would be needed to be manually checked on the screen for
correctness.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>