busybox

head: add test case for negative -n invocation

Commit 2da9724b56169f00bd7fb6b9a11c9409a7620981 broke 'head -n -1' and was
later reverted with 0d598ab9f03dbf320f7b81c05e4a94cb303dfbc7.

This commit adds a test case to avoid future breakage.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>