test command (in-)compatibility

[Andrei Borzenkov]

`test' in GRUB implicitly assumes `and' operation between consecutive
terms and does not enforce proper syntax like UNIX (bash) `test' does. Both

test x y z
test x = y z = w

result in error in Linux and are silently accepted by GRUB with
interpretation

test x -a y -a z
test ( x = y ) -a ( z = w )

I do not have any strong opinion about it; but simply documenting it
needs the least efforts :)