selftests/harness: Display signed values correctly

Since forever the harness output for signed value tests have reported
unsigned values to avoid casting. Instead, actually test the variable
types and perform the correct casts and choose the correct format
specifiers.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>