change expected exit values of tests to consistently be signed chars

change expected exit values of tests to consistently be signed chars

when a forked process finishes, waitpid() will return the status
as an int. Several macros determine what the status means.
WEXITSTATUS() is defined as returning the low-order 8 bits of the status
argument that the child process passed to _exit() or exit(), or the
value the child process returned from main(). It makes sense for the
value to be stored as either a signed or unsigned char. As the
decision is arbitrary (e.g. it only matters internally to check),
signed is used to keep consistent with the _tcase_add_test function.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1061 64e312b2-a51f-0410-8e61-82d0ca0eb02a