efi_selftest: Add box drawing character selftest

UEFI applications rely on Unicode output capability, and might use that
for drawing pseudo-graphical interfaces using Unicode defined box
drawing characters.

Add a simple test to display the most basic box characters, which would
need to be checked manually 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>