utiltest: Use int8_t instead of char.

Not every architecture out there has 'char' signed by default.
For instance, my arm box has it unsigned by default:

  $ gcc -dM -E - < /dev/null | grep __CHAR_UNSIGNED__
  #define __CHAR_UNSIGNED__ 1

Therefore, after 65c61e50 the test if failing for me. Problem is,
we are trying to assign couple of negative values into char
assuming some will overflow and some don't. That can't be the
case if 'char' is unsigned by default. Lets use more explicit types
instead: int8_t and uint8_t where is no ambiguity.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

tests/utiltest.c

@@ -184,8 +184,8 @@ static int
 testOverflowCheckMacro(const void *data ATTRIBUTE_UNUSED)
 {
     long long tmp;
-    unsigned char luchar;
-    char lchar;
+    uint8_t luchar;
+    int8_t lchar;
 
     TEST_OVERFLOW(luchar, 254, false);
     TEST_OVERFLOW(luchar, 255, false);