test: add tests for u16_str<n>cmp()

New seven test cases for u16_str<n>cmp() are added under Unicode unit test,
which should be executed by "ut unicode" command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>