Fix warnings in test_util_formats.

Storing 255 into a char gives a warning when char is signed.

Fixes bug 19682; bugfix on 0.2.8.1-alpha, where these tests were added.

changes/bug19682

@@ -0,0 +1,3 @@
+  o Minor bugfixes (compilation):
+    - Fix compilation warning in the unit tests on systems where
+      char is signed. Fixes bug 19682; bugfix on 0.2.8.1-alpha.

src/test/test_util_format.c

@@ -106,10 +106,10 @@ test_util_format_base64_encode(void *ignored)
   for (i = 0;i<50;i++) {
     src[i] = 0;
   }
-  src[50] = 255;
-  src[51] = 255;
-  src[52] = 255;
-  src[53] = 255;
+  src[50] = (char)255;
+  src[51] = (char)255;
+  src[52] = (char)255;
+  src[53] = (char)255;
 
   res = base64_encode(dst, 1000, src, 54, BASE64_ENCODE_MULTILINE);
   tt_int_op(res, OP_EQ, 74);