Invalid char comparison to negative value

[sam6258]

In tests like test_shmem_zero_get.c, there is an invalid comparison between a char and a negative number.

tests-uh/feature_tests/C/test_shmem_zero_get.c

Line 197 in 15d685d

if (dest8[i] != -9) {

Depending on the architecture's implementation of char, the test could fail. According to The C Programming Language, the implementation of char has implementation defined limits. The portable way to do this would be to use signed char.

Test cases failing on Power PC because of this.